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^^ Read Nature in the Language of Experiment,'" 



MODERN 



Electricity and Magnetism, 



Theoretically and Practically Considered. 



BEING 

THE CHAPTER ON ELECTRICITY, 

FROM 

THE ELEMENTS OF NATURAL PHILOSOPHY, 



J^3 



ELROY M. AVERY, Ph.D., 

AUTHOR OF A SERIES OF PHYSICAL SCIENCE TEXT-BOOKS. 






SHELDON AND COMPANY, 

NEW YORK AND CHICAGO. 



Dr. AVERY'S 
PHYSICAL SCIENCE SERIES. 



ist 

FIRST PRINCIPLES OF NATURAL PHILOSOPHY. 

2d. 

THE ELEMENTS OF NATURAL PHILOSOPHY. ^ 

3d- 
THE ELEMENTS OF CHEMISTRY. 

4th. 

THE COMPLETE CHEMISTRY. 

This contains the Elements of Chemistry, with an additional chapter on 
Hydrocarbons in Series or Organic Chemistry . It can be used in the same class 
with The Elements of Chemistry. 



Copyright^ 1885, by Sheldon df Company. 



"} 
V 



1 Electrotyped by Smith k McDgtjgal, 

82 Beekman St., New York. 






'i'^'i^ ^1^ J ^i^ ^1^ ^1^ 



THE rapid and remarkable deyelopments in Electrical 
Science made within the last ten years have been 
such as to attract the attention of the general public. 
Ocean and multiplex telegraphy, telephony, electric illu- 
mination^ electric railways, electric motors, electro-plating 
dynamos, time telegraph systems and the other countless 
applications of electricity have come in such rapid suc- 
cession, been so warmly welcomed and so largely accepted 
for common use, that the ^^ practical man of business" 
has been forced to look np from his balance sheet or 
market report to see what all of these things mean and, 
perhaps, even to ask, "What next?'^ Some of them have, 
it is rumored, been sufficiently interested in the matter to 
invest money in some of the enterprises for adapting 
this physical mystery to the doing of a part of the world's 
work. They and their children, their neighbors and their 
neighbors' children desire and need to know.more of the 
nature and peculiar eharacteristics of this old but feebly 



IT PREFACE, 

known acquaintance, who has so suddenly taken so active 
a part in the aflGairs of their every-day life. Hence, these 
pages. 

Yesterday, the few studied this subject, largely led by 
curiosity ; to-day, the many, impelled by business or 
scientific necessity. But the subject has grown and 
otherwise changed, as well as the class of students. In 
fact, the latter change is an effect of the former. " Elec- 
tricity" no longer covers merely a few disjointed phe- 
nomena, however interesting and remarkable; it has 
become a science. A treatise on the subject can no longer 
be made on the scrap-book plan ; it must constantly refer 
all things to an underlying principle, a stratum on which 
all is built and by which all is unified. 

Electricity is a department of physics and, in its 
largest sense, physics is the science of Matter and 
Energy. 



Intkomict©ky 



A S stated in the preface, electricity is a department 
f\ of physics, which is the science of matter and 
energy. 

Matter is anything that ^^ takes up room" or occu- 
pies space. It is always and everywhere subject to the 
attractive influence of a well-known force called gravita- 
tion. The two ideas of extension and weight are insep- 
arably united with the idea of matter. 

Atoms, molecules and masses are divisions of mat- 
ter. Atoms make molecules ; molecules make masses. 
Each of these has its own attraction and its peculiar 
forms of motion. Electricity and magnetism are, prob- 
ably, forms of molecular motion, as heat and light, cer- 
tainly, are. 

Scientific men, the world over, generally make use of 
the international or metric units of weights and measures. 
The unit of length is the meter (m.) and equals 39.3? 
inches — ^^a long yard.^^ It is divided successively into 
tenths, known as decimeters (dm,), centimeters (cm.) and 
milhmeters (mm.). The accompanying figure shows the 
decimeter (tenth-meter) and its subdivisions. The mul- 
tiples of all the metric units increase in a ten-fold 
ratio and are designated by the prefixes, deha-^ hehto-^ 



VI INTRODUCTORY CHAPTER. 



hilo-, etc. The kilometer (1000 m.) equals about 
0.62 of a mile. The divisions of all the metric 
units are designated by the prefixes deci-, centi-, 
milli-, A cubic centimeter {cu, cm,) of pure water 
at its temperature of greatest density furnishes the 
unit of weight and is called a gram [g.). A kil- 
ogram {Kg.) equals 2.2 lb. avoirdupois. A cubic 
decimeter furnishes the unit of capacity. It is 
called a liter (pronounced leeter) and, in value, is 
between the liquid and the dry quart. A liter of 
pure water weighs a kilogram. A milliliter {ml) 
of water, of course, weighs a gram. 
1 Force is a term used to designate any cause 



that tends to produce, change or destroy motion. 
Equal forces produce equal velocities when applied 
to equal bodies for equal times. The unit of force 
3 generally used in scientific work is the dyne. This 
is the force that, acting for one second on a mass 
of one gram, produces a velocity of one centi- 
meter per second. It is one of the "absolute" or 
"0. G. S.^' (centimeter-gram-second) units. The 
weight of a gram equals about 980 dynes. 
i^iG. I. Work is the overcoming of resistance. Work 
implies a change of position and is independent of the time 
taken to do it. The foot-pound is the unit of work most 
commonly used in this country, as yet. It is the work done 
in lifting a mass of one pound (avoirdupois) one foot 
high against the force of gravity. The work done in 
lifting a mass of one kilogram one meter high against 
the force of gravity is called a kilogram-meter. The 
C. G. S. unit of work is called an erg. It is the work 
done in moving a free body one centimeter against a force 



tNTRODUCTORY CHAPTER, vii 

of one dyne. The work of lifting one gram one centi- 
meter high against the force of gravity is about 980 ergs. 
A foot-pound is about 13,560,000 ergs. 

The rate of doing work is called power. A horse- 
power represents the ability to perform 550 foot-pounds 
in a second or 33,000 foot-pounds in a minute. It equals 
about 746 x 10'' ergs per second. 

Energy is the power of doing work. A mass of matter 
in motion is able to overcome resistance, or to do work, 
by virtue of its weight and velocity. Energy of motion, 
like that of a falling hammer, or of flowing water, is 
called kinetic energy. But matter often has a supply of 
possible energy by virtue of a position of advantage. 
Thus a bent bow, the raised weights of a clock or the 
water in a high reservoir, has a power of doing work by 
reason of its position alone. Such energy is called 
potential energy. It is evident that potential energy is 
easily converted into kinetic energy and vice versa. Both 
of these kinds of energy may pertain to masses of matter, 
as in the illustrations just given, or to the molecules of 
matter. The energy of vibrating molecules (molecular, 
kinetic energy) does the work of the steam engine and, 
in the case of the expansion of metals by heat, is practi- 
cally irresistible. We shall have much to do with molec- 
ular energy in the following pages. 

The various ideas, thus briefly discussed in this intro- 
ductory chapter, are more fully considered in the opening 
chapters of Averyh Elements of Natural Philosophy, to 
which the reader is respectfully referred. The rest of this 
book constitutes a chapter of the revised and enlarged 
edition of that work. The subject of Electrical Measure- 
ments is considered in the Appendix. 



«/^#, 



ELECTRICITY AND MAGNETISM 



Section l 



^ 



GENERAL VIEW. 

J^ote. — A desire to secure favorable atmospheric conditions for 
experiments in frictional electricity lias determined the order in 
which the following branches of physics are taken up. In most 
places in this country, the school-year begins with September. In 
such cases, this chapter would probably be reached by January, 
during" which month the atmosphere is generally dry. Under 
other circumstances, the consideration of these subjects would better 
be omitted until sound, heat and light have been studied. The 
experiments in this chapter are numbered consecutively. 

303. Simple Aj^paratus. — Provide two stout 
sticks of sealing-wax and one or two pieces of flannel folded 
into pads about 20 centimeters (8 inches) square; two 
glass rods or stout tubes closed at one end, 30 or 40 centi- 
meters in length and about 2 centimeters in diameter (long 
^•ignition tubes'' will answer) and one or two silk pads 
about 20 centimeters sqnare, the pads being three or four 
layers thick; a few pith balls about 1 centimeter in diam- 
eter (whittle them nearly round and finish by rolling 
them between the pa]ms of the hands) ; a silk ribbon 
about an inch wide and a foot long; a balanced straw 



184 



GENERAL VIEW, 



Fig. 119. 



about a foot long, represented in Fig. 119. The ends 
of the straw carry two small discs of paper (bright colors 
f^ ■ D preferable) fastened on by sealing-wax. 

The cap at the middle of the straw is 
a short piece of straw fastened by seal- 
ing-wax. This is supported upon the point of a sewing- 
nee41e, the other end of which is stuck upright into the 
cork of a small glass yial. From the ceiling or other con- 
venient support, suspend one of the pith balls by a fine 
silk thread. 

{a.) The efficiency of the silk pad above mentioned may be in- 
creased by smearing one side with lard and applying an amalgam 
made of one weight of tin, two of zinc and six of mercury. The 
amalgam that may be scraped from bits of a broken looking-glass 
answers the purpose admirably. 

Experiment I. — Draw the silk ribbon between two layers of the 
warm flannel pad with considerable friction. Hold it near the wall 
of the room. The ribbon will he drawn to the wall and held there for 
some time. Place a sheet of paper on a warm board and briskly 
rub it with india-rubber. Hold it near the wall as you did the ribbon. 

Experiment 2. — Briskly rub the sealing-wax with the flannel 
and bring the wax near 
the suspended pith ball. 
The ball will be drawn 
to the wax. Bring the 
wax near one end of the 
balanced straw ; it may 
be made to follow the 
wax round and round. 
Bring it near small 
scraps of paper, shreds 
of cotton and silk, 
feathers and gold leaf, 
bran and sawdust and 
other light bodies ; they 
are attracted to the wax. 

Experiment 3.— Repeat all of these experiments with a glass rod 
that has been rubbed with the silk pad. 




Fig. 120. 



GENERAL VIEW. 



185 



Experiment 4.— Make a light paper hoop or an empty egg-shell 
roll after your rod. (See § 332 h.) 

Experiment 5. — Place an egg in a wine-glass or an egg-cup, 
LTpon the egg, balance a yard-stick or a common lath. The end of 
the stick may be made to follow the 
rubbed rod round and round. Place the 
blackboard pointer or other stick in a 
wire loop (Fig. 121) or stiff paper stir 
rup suspended by a stout silk thread 
or narrow silk ribbon. It may be made 
to imitate the actions of the balanced 
straw or lath. 

Experiment 6. — Suspend the rubbed 
sealing-wax or glass rod as you did 
the blackboard pointer in the last ex- 
periment. Hold your hand near the 
end of the rod. It will turn round and approach your hand. 





Fig. 122. 



Note. — The pupil may be in- 
genious enough to invent new 
experiments for himself and 
the class. The ability to in- 
vent is often very valuable 
and may be acquired early m 
life. Most of the great in- 
ventors began making experi- 
ments when mere children. 

303. Electric At- 
traction. — The at- 
tractions Tnanifestecl 
in the experiments 
just described were 
due to electricity that 
was developed by fric- 
tion. Such electricity 



is called frictional or static electricity. 



) 



186 



GENERAL VIEW. 



Experiment 7. — Bring tlie rubbed sealing-wax or glass rod near 
the pith ball again. It will attract the ball as before. Allow the 
ball to touch the rod and notice that, in a moment, the ball is 
thrown off. If the ball be pursued with the rod, it will be found 
that the rod which attracted it a moment ago now repels it. Evidently, 
the ball has acquired a new property. (Fig. 123.) 

Experiment 8. — Touch the ball with the finger. It seeks the 
rubbed rod, touches the rod, flies from the rod. Repeat the experi- 
ments with the sealing-wax after it has been rubbed with flannel. 

Experiment 9. — Rub the glass rod with silk and bring it over 
the small scraps of paper as before. Notice that, after the attrac- 
tion, the paper bits do not merely fall down, they are thrown down. 



304. Electric 



Repulsion. — The repulsions 
manifested in the experi- 
ments just deseribed were 
due to static electricity. 
The glass or wax is said to be 
electrified by friction. The ball, 
after obtaining its new property 
of repulsion by coming in con- 
tact with the glass or wax, is said 
to be electrified by conduction. 
The suspended pith ball is 
called an electric pe7ididum. 



Experiment 10.— Prepare a battery 
solution according to the recipe given 
in § 892, using only half the quantity 
of each substance as therein directed. 
While the solution is cooling, provide a 
piece of sheet copper and one of sheet zinc, each about 10 centimeters 
(4 inches) long and 4 centimeters il^ inches) wide. To one end oi 
each strip, solder (se3 Appendix B) or otherwise fasten a piece ol 
No. 18 copper wire (See Appendix I) about 15 centimeters (6 inches) 
long. Place the zinc strip in a common tumbler about three-fourths 
full of the battery solution. Notice the minute bubbles that break 
away from the surface of the zinc and rise to the surface of the 




Fig. 123. 



GENERAL VIEW, 



187 



liquid. These are bubbles of liydrogen, a combustible gas. The 
formation of the gas is due to chemical action 'between the zinc and 
the liquid. 

Experiment II. — Take the zinc from the tumbler and, while it is 
yet wet, rub a few drops of mercury (quicksilver) over its surface 
until it has a brilliant, silver-like appearance. Replace the zinc, 
thus amalgamated, in the solution and notice that no bubbles are 
given off. 

Experiment 12. — Place the copper strip in the liquid, taking care 
that it or its wire does not touch the zinc or its 
wire. JVo bubbles appear either on the zinc or the 
copper. It may be convenient to place a narrow 
glass strip between the ends of the metal strips 
in the tumbler to keep them apart. 

Experiment 13. — Bring the upper ends of the 
strips together, as shown in Fig. 124, or, still 
better, join the two wires, as shown in Fig*. 179, 
being sure that the wires are clean and bright 
where they are united. Notice the formation of 
bubbles on the surface of the copper, where none 
previously appeared. 




Fig. 124. 



305. Suspicion. — It seems that the connecting 
wire is an important part of the apparatus as now ar- 
ranged and we are led to suspect that something unusual 
is taking place in the wire itself. It is evident that we 
have a complete ^^ circuit'^ through the liquid^ the metal 
strip and the wire. 

Experiment 14.— Untwist the wires or, in other words, '' break 
the circuit." Connect the copper wires with a short piece of very 
fine iron wire. The connections should be made so that the circuit 

shall include about 2 centimeters 
(f inch) of iron wire. The iron 
icill become hot enough to burn the 
fingers or to ignite a small quantity 
of gun cotton twisted around it. 




Fig. 125. 



Experiment 15. — If one of the 

copper wires be twisted around one 
end of a small file and the free end 



188 GENERAL VIEW, 

of the other wire be drawn along its rough surface, a series of 
minute sparks will he produced as the circuit is rapidly made and 
broken. 

Experiment 16. — Place the cell so that the joined wires shall run 
north and south, passing directly over the needle of a small com- 
pass (Experiment 98) and near to it. The needle will instantly turn as 
though it were trying to place itself at right angles to the wire. 
Break the circuit and the needle will swing back to its north and 
south position. 




Fig. 126. 



306. Certainty. — We now feel sure that something 
unusual is taking place in the wire of our complete circuit, 
for we have seen the wire become hot, explode gun-cotton, 
yield sparks and exert a very mysterious influence upon 
the magnetic needle. As a matter of fact, we now have 
a current of electricity flowing through a voltaic cell and 
wire. Electricity thus -produced by chemical action 
is called voltaic or galvanic electricity. It is one 
form of current electricity. 

Experiment 17. — Wrap a piece of writing paper around a large 
iron nail, leaving the ends of the nail bare. Wind fifteen or twenty 
turns of stout copper wire around this paper wrapper, taking care 
that the coils of the wire spiral do not touch each other or the iron. 
It is well to use cotton covered or 'Mnsulated" wire. Connect the 
two ends of the wire spiral with the two wires of the voltaic cell 



GENERAL VIEW. 189 

or, in other words, put the spiral into the circuit. Dip the end of 
the nail into iron filings. Some of the filings will ding to the nail in 
a remarkable manner. Upon breaking the circuit, the nail instantly 
loses its newly acquired power and drops the iron filings. 

If the experiment does not work satisfactorily, look carefully to 
all the connections of the circuit, see that the ends of the wires are 
clean and bright and that they are twisted together firmly. It may 
be necessary to wash the plates, rub more mercury on the zinc and 
provide a fresh battery solution. 

307. Temporary Magnets. — The nail has the 
power of attracting iron filingvS luhile the electric citr- 
rent is fioiving through the siorj^ounding wire coil. 
You have made aiv electro-magnet. Its poiver of 
attracting iron is called magnetism. Satisfy your- 
self, by trial, that the nail loses its magnetism as soon as 
the circuit is broken or the current ceases to flow around 
it. Remember that your electro-magnet is a temporary 
magnet. 

Experiment 18. — While the nail is magnetized, draw a sewing- 
needle four or five times from eye to point across one end of the 
electro-magnet. Dip the needle into iron filings ; some of them vnll 
cling to each end of it. 

308. Permanent Magnets. — When steel is 
treated as in the last experiment^ it becomes permanently 
magnetized. 

Experiment 19. — Cut a thin slice from the end of a vial cork and, 
with its aid, float your magnetized needle upon the surface of a 
bowl or saucer of water. The needle comes to rest in a north and 
south position. Turn it from its chosen position and notice that, after 
each displacement, it resumes the same position and that the same 
end of the needle always points to the north. 

.309. A Sini|)le Compass.— e^ small magnet- 
ized steel bar freely suspended, is called a com- 



190 GENERAL VIEW. 

pass. The one that you have made may be less conven- 
ient than is the compass of the mariner or the surveyor, 
but it is as reliable. 

310. Artificial Magnets. — The electro-magnet 
and the permanent magnet that you make are, of course, 
artificial magnets. There is a natural magnet 
known as lodestone. 

311. Otlier Forms of Current Electricity. — 

Electric currents may be generated by the action of other 
currents of electricity or by tlie action of magnets. Elec- 
tricity thus developed is called induced electricity, A 
current of tliermo-electricity may be generated by heating 
the junction of two metals that form part or all of a cir- 
cuit. 

313. The Different Forms of Electricity 
are Identical. — So far as experiment can show^ one 
form of electricity may have a particular property in 
greater degree than some other form, but all are identical, 
each having all the properties of any of the others. 



GENERAL VIEW, 



191 



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ECTION n. 



FRICTIONAL ELECTRICITY OR ELECTRIC CHARGES. 

313. The Nature of Electricity —But little is 
known concerning the real nature of electricity. It is 
easier to tell what electricity can do than to tell what it 
is. The majority of modern physicists consider that elec- 
tricity is a form of energy producing peculiar 
phenomena ; that it may he converted into other 
forms of energy and that all other forms of 
energy may he converted into it. It is belieyed that 
electricity is a form of molecular motion, but this belief 
still rests upon analogy rather than demonstration. Sev- 
eral theories have been advanced to account for electrical 
phenomena, but none of them is satisfactory. 

314. Electric Manifestations. — Electricity 
may reveal itself as a charge residing on tlie sur- 
face of a hody or as a current floiuing through its 
suhstance. By means of friction, the glass rod or the 
sealing-wax (§§ 303, 304) acquired an electrical charge 
and, consequently, the power of attracting and repelling 
light bodies ; by means of chemical action, the voltaic cell 
(§ 306) generated electricity that manifested itself as a 
current. In this section, we shall consider electricity that 
appears as a charge, i.e., static electricity. 



FRICTION AL ELECTRICITY, 



193 



(«.) The electrified body is said to be charged. When the electric- 
ity is removed, the body is said to be discharged. Good conductors 
(§ 324) are instantly discharged when touched by the hand, or by any 
good conductor connected with the earth. A poor conductor may 
be readily discharged by passing it rapidly through a flame, as of a 
lamp or candle. 

Experiment 20. — Prepare two electric pendulums. Bring the 
electrified glass rod near the pith ball of one ; after contact, the ball 
will be repelled by the glass. Bring the electrified sealing-wax 
near the second pith ball ; after contact, it will be repelled by the 
wax. Satisfy yourself that the electrified glass will repel the first ; 
that the electrified sealing-wax will repel the second. Let the glass 
rod and the sealing-wax change hands. The first ball w as repelled 
by the glass ; it will be attracted by the sealing-wax. The second ball 
was repelled by the sealing-wax ; it will be attractedJyy the glass. 

Experiment 21. — Suspend two pith balls as shown in Fig. 127, 
and touch them with a rubbed 
glass rod. Instead of con- 
tinuing to hang side by side, 
they repel each other and fly 
apart. If the electrified glass 
rod be held near them, they 
separate still further. If 
the electrified sealing-wax, 
instead of the glass, be held 
near them, they will fall 
nearer together. If the 
rubbed glass rod be sus- 
pended as shown in Fig. 121, 
it will be repelled by another 
rubbed glass rod, but at- 
tracted by rubbed sealing- 
wax. 




Fig. 127. 



315. Two Kinds of Electricity. — I7i^ elec- 
tricity developed on glass is different in kind from 
that developed on sealing-iuax. They exhibited op- 
posite forces to a third electrified body, each attracting 
what the other repels. 




194 FRlCTIONAL ELECTRICITY. 

Experiment 22. — Hold tlie silk pad in a piece of slieet-rubber 
and, with it, rub the glass rod. Suspend the glass rod and bring 

the silk pad near it. The electrified 
pad will attract the glass, but will 
repel a suspended stick of sealing-wax 
that has been rubbed with flannel. 

316. Electric Separa- 
tion. — All electrified bodies 
act like either the glass or the 
sealing-ivax. When the glass 
rod was positively electrified, an 
P g equal amount of negative elec- 

tricity was simultaneously devel- 
oped in the silk with which it was rubbed. When the seal- 
ing-wax was negatively electrified, an equal amount of 
positive electricity was developed at the same time in the 
flannel. It is as though the two electricities were united 
in these several substances in their ordinary condition and 
were torn asunder by the friction, thus producing actual 
" electric separation. ^^ 

{a.) If it be desired to show that the rubber has been electrified, 
care must be taken not to handle it too much. For example, if seal- 
ing-wax is to be rubbed with a piece of fur, do not take the fur in 
the hand, but fasten it to the end of a glass rod as a handle. 

(&.) That the electricities thus simultaneously developed are op- 
posite in kind and equal in amount may be shown by imparting 
the electricity of the rubber and the electricity of the thing rubbed 
to a third body, which will then show no electrification at all. The 
equal and opposite electricities exactly neutralize each other. 

317. The Two Electricities Named.— As the 

two kinds of electricity are opposite in character, they 
have received names that indicate opposition. The elec- 
tricitrf developed on glass hij rubbing it with silJc 



FRICTIONAL ELECTRICTTY. 195 

is called positive or +. The electricity developed 
on sealing-iuajo by rubbing it with flannel is called 
negative or — . The terms vitreous and resinous 
respectively were formerly used. 

318. Electric Series. —In the following list, the substances 
are named in such an order that, if any two be rubbed together, the 
one that stands earlier in the series becomes positively electrified 
and the one that is mentioned later becomes negatively electrified : 
fur, wool, resin, glass, silk, metals, sulphur, india-rubber, guttapercha, 
collodion. 

319. The Laws of Electrostatics.— The most 
important electrostatic laws may be stated thus : 

(1.) Electric charges of like signs repel each other ; 
electric charges of opposite signs attract 
each other, 

(2. ) TJxe force eocerted between two electric charges 
is directly proportional to their product 
and inversely pi'oportional to the square 
of the distance betiueen them. This is known 
as Coulomb^s law. The two charges are sup- 
posed to be collected at two points^ or on two 

very small spheres. / = — ^^• 

{a.) Suppose that a and b are two small balls, each charged with 
a quantity of electricity, that w^e shall call unity. Then the product 
of the charges will be 1 xl— 1. Next, suppose that A and B are 
two similar balls, that A is charged with twice as much electricity as 
a and that, similarly, B has a charge represented by 3. The prod- 
uct of the charges of A and B will be 2 x 3== 6. In other words, at 
equal distances, the repulsion between A and B will be six times as 
great as the repulsion between a and b. 

(b.) Suppose that two electric charges or two small electrified 
bodies one inch apart repel each other with a certain force : at a dis- 
tance of two inches, they will repel each other with a force one quarter 
as ^reat ; at a distance of ten inches, they will repel each other with 
only one per cent, of the original force at the distance of one inch. 



196 FRICTIONAL ELECTRICITY, 

320. Electrical Units. — There are two systems of 
electrical units derived from the fundamental '^C.G.S/'" 
units, one set being based upon the attraction or repulsion 
exerted between two quantities of electricity and the 
other upon the force exerted between two magnet poles. 
The former are termed electrostatic units ; the latter, elec- 
tromagnetic units. 

331. Electrostatic Unit of Quantity. — One 

unit of ^electricity is that quantity which, ivhen 
placed at a distance of one centimeter from a 
similar and equal quantity , repels it with a force 
of one dyne. It is a C.G.S. unit (§ 69) and has no 
special name. 

(a.) Two small spheres, charged respectively with 6 units and 8 
units of + electricity, are placed 4 cm. apart ; find what force they 
exert on one another. 

By the formula, / = 9jll, we find / = ^ = ^ = 3. 

An.9. 3 dynes. 

The force in the above example would clearly be a force of repul- 
sion. Had one of these charges been negative, the product, § x g, 
would have had a — value (algebraic) and the answer would have 
been minus 3 dynes. The algebraic — sign, therefore, prefixed to 
a force, indicates that it is a force of attraction, while the 4- sign 
signifies a force of repulsion. 

332. The Test for Either Kind of Elec- 
tricity. — When the pith ball was attracted by the rubbed 
glass it became^ during the time of contact, charged with 
the + electricity of the glass; hence it was repelled. 
When it was attracted by the rubbed sealing-wax it be- 
came, during the time of contact, charged with the — 
electricity of the wax ; then it was repelled. But either 



FRICTIONAL ELECTRICITY. 



197 



the wax or the glass attracted the uncharged pith ball. 
We must, therefore, remember that attraction affords 
no safe test for the kind of electricity, while re- 
pulsion does. If glass rubbed with silk repels a body, 
that body is charged with -f electricity. If sealing-wax 
rubbed with flannel repels a body, that body is charged 
with — electricity. 

333. Electroscopes. — An instrument used to 
detect the presence of electricity, or to determine 
its hind, is called an electroscope. The electric pen- 
dulum (§ 304) is a common form of the electroscope. 
Two strips of the thinnest tissue paper hanging side by 
side constitute a simple electroscope. It is well to prepare 
the paper beforehand by soaking in a strono: solution of 
salt in water and drying. 
The balanced straw (Fig. 
119) or, better yet, two 
gilded pith balls connected 
by a light needle of glass 
or sealing-wax balanced 
horizontally on a vertical 
pivot, or a goose-quill 
balanced on the point of 
a sewing-needle, makes a 
convenient electroscope. 

The gold leaf electro- 
scope is represented in 
Fig. 129. A metallic rod, which passes through the cork 
of a glass vessel, terminates below in two narrow strips of 
gold leaf and above in a metallic knob or plate. The 
object of the vessel is to protect the leaves from disturb- 
ance by air currents. The upper part of the glass is often 




Fig. 129. 



198 FRICTIONAL BLECTRIClTY. 

coated with a solution of sealing-wax or shellac in alcohol, 
to lessen the deposition of moisture from the atmosphere. 
This instrument may be made by the pupil and, when 
well made, is yery dehcate. 

{a,) The electric pendulum is used as an electroscope as follows : 
If an uncharged pith ball be attracted by a body brought near it, 
the body is electrified. To determine the sign of the electricity of 
the body thus shown to be electrified, the pith ball is allowed to 
touch it and be repelled. If the ball then be repelled by a glass rod 
rubbed with silk (or by any other body known to be positively 
charged), the pith ball and the body in question manifest + elec- 
tricity. If the pith ball, after repulsion by the body whose elec- 
tricity is under examination, be repelled by sealing-wax rubbed with 
flannel (or by any other body known to be negatively charged), the 
pith ball and the body in question manifest — electricity. Remem- 
ber that the repulsion and not the attraction constitutes the test. 

(&.) One way of testing with the gold leaf electroscope is to bring 
the electrified body near the knob ; the leaves will diverge. Touch 
the knob with the finger ; the leaves will fall together. Remove first 
the finger and then the electrified body ; the leaves will diverge 
again. If now the divergence of the leaves be increased by bring- 
ing a positively charged body near the knob, the original charge 
was — ; if the divergence be thus diminished, the original charge 
was +. 

(c.) The knob and rod of the gold leaf electroscope may be made 
by soldering a wire to a smooth metal button. The vessel may be 
any clear glass bottle with a wide mouth. Thrust the wire down- 
ward through the cork of the bottle and bend the wire at right 
angles, so that when the cork is in place the horizontal part of the 
wire shall be about f inch long and come just below the shoulder of 
the bottle. Cut a strip of gold or Dutch leaf, 4 inches long and \ 
inch wide and paste it at its middle line to the horizontal part of the 
wire, so that the two halves of the strip shall hang downward facing 
each other. See that the cork is perfectly dry ; heat the bottle until 
it is perfectly dry ; insert the cork firmly in its place, and pour 
melted sealing-wax over the cork and around the mouth of the 
bottle so that no moisture can get into your electroscope. If you- 
cannot get the gold or Dutch leaf (try at some good-natured dentist's 
or sign painter's), use two discs of gilt paper as large as the mouth 
of your bottle will admit and tie them to the wire by very short 
cotton or linen threads. 



FRICTIONAL ELECTRICITY. 



199 



Experiment 23. — From a liorizontul glass rod or tightly-stretched 
silk cord, suspend a fine copper wire, a linen thread and two silk 
threads, each at least a meter long. To the lower end of each, at- 
tach a metal weight of any kind. Place the weight supported by 
the wire upon the plate of the gold leaf electroscope. Bring the 
electrified glass rod near the upper end of the wire ; the gold leaves 
instantly diverge. Repeat the experiment with the linen thread ; in 
a little while the leaves diverge. Repeat the experiment with the 
dry silk thread ; the leaves do not diverge at all. Rub the rod upon 
the upper end of the silk thread ; no divergence yet appears. Wet 
the second silk cord thoroughly and, with it, repeat the experiment ; 
the leaves then diverge instantly. 

Experiment 24. — Support a yard stick or common lath upon a 
glass tumbler. Bring the glass rod, electrified by rubbing it with 
silk, to one end of the stick and hold some small pieces of gold leaf 
or paper under the other end of the stick. The gold leaf or paper 
will be attracted and repelled by the stick as it 'previously was by 
the glass itself. The electricity passed along the stick from end to end. 



334. Conductors. — Such experiments clearly show 
that soine substances transmit electricity i^eadily and 
that others do not. Those that offer little resistance 
to the passage of electricity are called conductors ; 
those that offer great resistance are called non- 
conductors or insulators. A conductor supported by a 
non-conductor is said to be insulated. 

(a.) In the following table, the substances named are arranged in 
the order of their conductivity : 



Conductors. 
\. Metals, 

2. Charcoal. 

3. Graphite. 

4. Acids. 



5. Salt water. 

6. Fresh water. 

7. Vegetables. 

8. Animals. 

9. Linen. 



10. Cotton. 

11. Dry wood. 

12. Paper. 

13. Silk. 

14. India rubber. 



15. Porcelain. 

16. Glass. 

17. Sealing-wax. 

18. Vulcanite. 
Insulators. 



(b.) The fact that a conductor in the air may be insulated, shows 
that air is a non-conductor. Dry air is a very good insulator (at 
least 10^*^ times as good as copper), but moist air is a fairly good 
conductor for electricity of high potential. All experiments in fric- 
tional electricity should, therefore, he performed in clear, cold weather 



300 FRICTIONAL ELECTRICITY, 

when the atmosphere is dry, for a moist atmosphere renders insula- 
tion for a considerable length of time impossible. 

(c.) A simple way of determining experimentally whether a body is 
a good conductor or not is, to hold it in the hand and touch the knob 
of a charged gold leaf electroscope with it. If the substance be a 
good conductor, the electroscope will be quickly discharged. 

Experiment 25. — Suspend a copper globe or other metal body by 
a silk thread and strike it two or three times with a cat's skin or 
fox's brush. Bring the gold leaf electroscope near the globe. The 
leaves will diverge. 

335. Electrics. — Any substance, when insulated, 
may he sensibly electrified ; but when an uninsulated 
conductor is rubbed, the electricity escapes as fast 
as it is developed. The old division of bodies into elec- 
trics and non-electrics, or bodies that can be electrified 
and those that cannot be electrified, is nothing more than 
a division into conductors and non-conductors. 

326. Tension. — Electricity exists under widely dif- 
ferent conditions witii respect to its ability to force its 
way through a poor conductor or to leap across a gap. 
The electricity developed in a voltaic cell will not pass 
through even a very thin piece of dry wood ; the elec- 
tricity developed by rubbing the glass rod will pass through 
several feet of dry wood. It would require a battery of 
many cells to force a current across an air-filled gap of 
-Yio-Q of an inch. It is not difficult to force frictional 
electricity across a gap of several inches, while we all know 
that, in the case of lightning, electricity leaps across a 
gap of many hundred feet. In the one case, the electricity 
is said to be of low potential ; in the other case, it is said 
to be of high potential. The terms ^'low tension" and 
-^ " high tension " are often used in the same sense. 



FRICTIONAL ELECTRICITY. 201 

337« Potential. — The term, electrical potential (or 
simply potential), has reference to the electrical condition 
of a body, or to its degree of electrification. If the poten- 
tial of ^ be higher than that of B and the two bodies be 
connected by a good conductor, an electric current 
will flow from A to B until the potentials are 
alike. Difference of potential is somewhat analogous to 
difference of liquid level and gives rise to electromotive 
force. 

iji,) The electric condition of the earth is sometimes taken as the 
zero of potential. The electric condition of other bodies is then 
described as being a certain number of units above or belovr zero ; 
i.e., as being + or — . In determining the flow of liquids, it is not 
necessary to know the height of either reservoir above the earth's 
centre or above the sea level, but only the head or difference of 
liquid level. Similarly, the difference of potential is what determines 
the direction and strength of an electric current flowing through a 
given conductor. 

338. Difference of Potential.— 17^^ difference 
of potential between two points represents the ivorh 
that must he clone in carrying a + unit of electricity 
(§321) from one point to the other. The work done 
will be the same, whatever the path along which the unit 
is moved from one point to the other. Similarly, the 
work done in lifting a weight from one point to another 
at a higher level will be the same whatever the path along 
which the weight is lifted. 

339. Electrostatic Unit of Difference of 
Potential. — Tl%e unit of difference of potential is 
that which exists hetween two points, it/lien it re- 
quires the expenditure of one erg to bring a unit 
of + electricity from one point to another against 



202 FRICTIONAL ELECTRICITY. 

the electric force. Let ^ be a small sphere positively 
electrified and P and Q, two points at diflPerent distances 

from A, If Q is just so far 

y'" '"--.^ from P that it requires one erg 

,. ,^^ ^\ of work to push a unit of + 

electricity from Q to P, there 
! i • t ^ will be unit difference of poten- 
tial between P and Q. This 
" ' / unit has no special name. 



'" " {a.) Let P and Q be in the outer 

Fig. 130. surfaces of concentric, spherical, shells 

at the centre of which is A. To move 
the + unit from one point in either of these surfaces to any other 
point in the same surface requires no further overcoming of elec- 
tric forces and, therefore, no expenditure of work. Such a surface 
is called an equipotential surface. 

330. Electric Cai)acity. — Bodies vary in respect 
to their capacity for holding or accumulating electricity. 
The electrostatic unit of capacity is the capacity 
of a conductor that requires a charge of one unit 
of electricity to raise its potential from zero to 
unity. It has no special name. A sphere of one centi- 
meter radius has unit capacity. The capacities of spheres 
are proportional to their radii. (See § 359.) 

{a) A small conductor {e.g., a sphere the size of a pea) will require 
less than one unit to raise its potential from to 1 ; it is of small 
capacity. A sphere five meters in diameter will require many units 
to raise its potential from to 1 ; it is of great capacity. In other 
words, the electrostatic capacity of a conductor or condenser is 
measured by the quantity of electricity which must be imparted to 
it in order to raise its potential from to 1. 

331. Charging by Contact.— If an insulated, un- 
electrified conductor be brought into contact with a simi- 



FRICTIOXAL ELECTRICITY, 



203 



lar conductor that is electrified^ or near enough to it for 
the easy passage of an electric spark, electricity will pass 
from the latter to the former until the two conductors are 
equally charged with the same kind of electricity, i,e,, un- 
til they are of the same potential. The former is said 
to he ehargecl by conduction. 

333, Electrostatic Induction.— From several of 
the precedhig experiments, we see that actual contact with 
an electrified body is 
not necessary for the 
manifestation of electric 
action in an unelectri- 
fied body. When an 
electrified body, (7, is 
brought near an insu- 
lated, unelectrified con- 
ductor, B, provided 
with electric pendu- 
lums, as shown in Fig. 131, the latter shows electric ac- 
tion. The electricity of C repels one kind of electricity 
in B and attracts the other, thus separating them. The 
second body, B, is then said to be polarized. 

The two kinds of electricity in B, each of which a mo- 
ment ago rendered the other powerless, are still there, but 
they have been separated and each clothed with its proper 
power. This efiect is due to the action of the electrified 
body, (7, which is said to produce electric separation ly 
induction. This action will take place across a consider- 
able distance, even if a large sheet of glass be held be- 
tween B and C. When C is removed, the separated elec- 
tricities of B again mingle and neutralize each other. 




Fig. T31. 



304 



FRICTIONAL ELECTRICITY. 







Fig. 132. 



{a.) Conductors for the purposes of this and similar experiments 
may be made of wood, covered with tin-foil, gold leaf or Dutch 
leaf. They may be insulated by fastening them on top of long- 
necked bottles or sticks of sealing-wax, or by suspending them by 
silk threads. 

(6.) Prick a pin-hole in each end of a hen's egg and blow out the 
contents of the shell. Paste tin-foil or Dutch leaf smoothly over 
the whole surface of the egg. Fasten one end of a white silk thread 

to the egg with a drop 
^- '■ ' ^ III ' ' of melted sealing-wax, 

so that the egg may 
hang suspended with its 
greater diameter hori- 
zontal. Three or four 
such insulated conduc- 
tors will be found con- 
venient. Sometimes, it 
is better for each egg to 
have two thread sup- 
ports. Place a loop or 
ring at the free end of 
each thread. When the loops are placed on a horizontal rod {e.g., a 
piece of glass tubing), the greater diameters of the suspended eggs 
should lie in the same straight line. An elongated conductor like 
AB of Fig. 133 may be made by hanging two or three egg con- 
ductors, so that they are in contact, as shown in Fig. 132. 

Experiment 26. — While the charged glass rod is held near the 
egg conductors, shown in Fig. 132, bring a pith ball electroscope 
near. The attraction will be evident at the free ends of the two 
eggs, but very little, if any, will be found at or near the point 
where the eggs are in contact. 

333. A Neutral Line. — If an insulated conductor, 
bearing a number of pith ball (or paper) electroscopes^, be 
brought near an electrified body, C, (Fig. 133), but not 
near enough for a spark to pass between them^ the pith 
balls near the ends of the conductor will diverge^ showing 
the presence of separated or uncombined electricity. The 
pith balls at the middle of the polarized conductor will not 
diverge^ marking thus a neutral line. If C has a positive 



FRICTIONAL ELECTRICITY. 



205 



charge, the charge at A will be negative and that at B 
will be positive, as may be shown by charging an electric 
pendulum and testing at A and B, 




Fig. 133. 

If be removed or " discharged ^^ by touching it with 
the liand, all traces of electrical separation in ^ ^ will 
disappear. The charged pith ball will be attracted at 
every point of A B, 

Experiment 27. — While the charged glass rod is held near the 
^g^ conductors shown in Fig. 132, slide the loop, carrying A about 
4 inches (10 cm.) to the left and then hold the rod between the two 
eggs. The rod will repel one egg and attract the other. 



334. Charging a Body by Indviction.— If the 

polarized conductor be touched with the hand, or other- 
wise placed in electric communication with the earth, the 
electricity repelled by C (Fig. 133) will escape, and the 
pith balls at B will fall together. The electricity at the 
other end will be held by the mutual attraction between 
it and its opposite kind at O. The line of communica- 
tion with the ground being broken and the conductor 



306 



FRICTIONAL ELECTRICITY, 



being removed from the vicinity of (7, it will be found 
charged with electricity opposite in kind to that of C. 

A hocly may he thus charged by induction with 
no loss to the inducing body. If the conductor, AB, 
be made in two parts and the parts separated, while 
under the inductive action of the electrified body, C, the 
two electricities can no longer return to neutralize each 
other, but must remain, each on its own portion of the 
conductor. The two parts will thus be oppositely charged. 

335. Successive Induction.— If a series of insu- 
lated conductors, like the egg shells of Fig. 132, be placed 
iu line as shown in Fig. 134, and a positively electrified 




Fig. 134. 

body be brought near, each conductor will be polarized. 
The first will be polarized by the influence of the + of 
C; the second by the influence of the + of If, and so on. 

(a.) Either kind of electricity may be carried from Jf or iV^ by a 
small insulated body, called a proof-plane (Fig. 139), to tlie elec- 
troscope, there tested and found to be as represented in the figure. 
If the conductors, if and W, be now placed in actual contact, the + 
of both will be repelled by C to the furthest extremity of iV and 
the — of both will be attracted to the opposite end of M, near to 0, 



FRICTIONAL ELECTRICITY. 207 

(6.) It is very plain that any body may be looked upon as a collec- 
tion of many parallel series of such conductors, each molecule rep- 
resenting a conductor. Thus, each molecule may be polarized, -f 
at one end and — at the other. If the body in question be a good 
conductor of electricity, this polarization of the molecules is only for 
an instant. The two electricities pass from molecule to molecule 
and accumulate at opposite ends of the body. The body is then 
polarized, but not the molecules of the body. On the other hand, 
good insulators resist this tendency to transmit the electricities from 
molecule to molecule and are able to maintain a high degree of 
molecular polarization for a great length of time. In brief, the 
molecules of conductors easily discharge their electricities into each 
other ; those of non-conductors do not. 

336. Polarization Precedes Attraction, — 

When an electrified glass rod is brought near an electric 
pendulum, the pith ball is polarized 
as shown in the figure. As the — 
electricity of the ball is nearer the + 
of the glass than is the + of the ball, 

the attraction is 2Teater than the re- 

FiG. 135. 
pulsion. If the pith ball be sus- 
pended, not by a silk thread but by some good conductor, 
the attraction will be more marked, for the + of the ball 
will escape to the earth through the support and, thus, the 
repelling component will be removed. 

Note. — Polarization and electrification by induction explain a great 
many electrical phenomena. 

337, Provisional Theory of Electricity.— 

While the real nature of electricity remains unknown, the 
following theory will be found convenient for classifying 
results already attained and suggesting directions for fur- 
ther inquiry. But we must not let it influence our judg- . 
nient as to what is the true and fnll explanation of elec- 




208 FRICTIONAL ELECTRICITY, 

trical phenomena, which explanation may be found here- 
after : 

(1.) We may assume that a neutral or uneCectri- 
fied body contains equal and equally dis- 
tributed quantities of positive and of nega- 
tive electricity. 

(2.) We may assume these electricities to be un- 
limited in amount, 

(3.) We shall then conceive that a positively elec- 
trified body has an excess of + electricity 
and that a negatively electrified body has 
an excess of — electricity, 

(4.) In this light, we shall see that communi- 
cating 4- electricity to a body is equiva- 
lent to reinoving an equal amount of — 
electricity from it, and conversely. 

338. The Electrophorus.— This simple instru- 
ment consists generally of a shallow tinned pan filled with 
resin, on which rests a movable metallic cover with a glass 
or other insulating handle. The resinous plate may be 
replaced by a piece of vulcanized india-rubber. The metal 
surface and the resinous surface touch at only a few 
points; they are practically separated by a thin layer of 
insulating air. 

(a.) The resinous plate may be prepared by melting together 
equal quantities of resin and Venice turpentine and then adding a 
like quantity of shellac. The substances should be heated gradually 
and stirred together so as to prevent the forming of bubbles. Be 
careful that the mixture does not take fire in course of preparation. 
The Venice turpentine is desirable, but not necessary. For a handle, 
a stout wire may be soldered to the centre of the disc and covered 
with rubber tubing, or a piece of sealing-wax, of convenient size, 



FRICTIONAL ELECTRICITY. 



209 




may be fastened to the disc for the purpose. A still better plan is 
to make the cover of wood, a little less in diameter than the resinous 
plate. Its edges should be carefully rounded off. For a handle, a 
glass rod or tube may be tightly 
thrust or cemented into a hole in 
the middle of the cover. Place tin- 
foil all over the cover and smooth 
down all rough ed es of the foil 
with the finger-nail or paper-folder. 
The wire support for a pith ball or 
paper electroscope may be thrust 
into the wood of the cover, care be- 
ing taken that it touches the tin- 
foil. 

(&.) For an electroscope for the 
electro phorus, provide a bit of wire 
about 8 cm. long and bend it at 
right angles about 1 cm, from each 
end. Solder one of the bent arms 
of the wire (see Appendix B) to 

the upper side of the metal cover, near its edge, in such a way that 
the central part of the wire shall be vertical. Cut a strip of gold 
leaf (or Dutch metal) about 8 cm, long and 8 mm. wide. Moisten 
the sides of the free horizontal wire-arm with a little mucilage, 
place the middle of the gold-leaf strip over the top of the arm and 
bring the ends of the leaf down to a vertical position, touching each 
other. The mucilage will hold the leaf to the wire. When the 
wire support and gold leaves are electrified, the latter will diverge. 
When the a})paratus is not in use, this electroscope may be protected 
by inverting a tumbler or beaker glass over it. 

(c.) The plate is rubbed or struck with flannel or catskin and 
thus negatively electrified. The cover is then placed upon the resin 
and thus polarized by induction. If the cover be provided with a 
gold-leaf electroscope, the free negative electricity of the cover will 
cause the leaves to diverge ; the positive electricity of the cover 
will be " bound " on the under side of the cover by the attraction 
of the negative electricity of the resin. Remove the cover and the 
separated electricities reunite, as is shown by the falling together of 
the lately divergent gold leaves. Place the cover again upon the 
resin. Polarization is manifested by the divergence of the leaves. 
Touch the cover with the finger as shown in the figure ; the ~ 
electricity escapes and the leaves fall. The cover is now charged 
positively, but its electricity is all "bound" at its under surface 



210 



FRICTIONAL ELECTRICITY, 



and cannot cause the leaves to separate. Remove the cover by its 
insulating handle and the electricity, lately "bound" but now 
"free," diffuses itself and the leaves are divergent with + elec- 
tricity. The charged cover will give a spark to the knuckle or other 
unelectrified body presented to it. (Fig. 137.) 




339. The Electrophorus Charged by Induc- 
tion. — The cover may 
be thus charged and 
discharged an indefi- 
nite number of times, 
in favorable weather, 
Avithout a second elec- 
trifying of the resinous 
plate. This could not 
happen if the electricity 
of the cover were drawn 
from the plate. More- 
over, if the charge of 
the cover were drawn 
from the plate, it would 
be ■— , and not +. 
There is no escape from 
the conclusion that the 
cover is charged by induction and not by conduction. 




Fig. 137. 



(a.) If the resin were a good conductor like the metal cover, its 
molecules would all receive + electricity from the cover and give 
— electricity to it. But as the resin is a poor conductor, only the 
very few molecules that come in actual contact with the cover at 
each charging have their electrical equilibrium restored. The + 
of the cover cannot readily pass through them to their electrified 
neighbors. Hence, it requires a great many placings of the cover 
upon the plate to discharge the resin by reconveying to it the -f 
electricity removed at its electrification. When the cover is charged, 
it gives up part of its — electricity ; when it is discharged, it re- 



FRICTIONAL ELECTRICITY. 



211 



ceives this — electricity- back again from the body that discbarges 
it. As this giving and taking is neither to nor from the resin, it 
maybe continued almost indefinitely. A Leyden jar (§ 353) may be 
charged with an electrophorus. 

340. Whence this Energy?— At every discharge 
of the electrophorus, it gives a definite amount of elec- 
tricity, capable of doing a definite amount of work. As 
this is obtained not by the expenditure of any part of the 
original charge, we are led to seek for the source of this 
apparently unlimited supply of energy, 

" As a matter of fact, it is a little harder work to lift 
the cover when it is charged with the + electricity than 
if it were not charged, for, when charged, there is the 




Fig. 138. 



force of electric attraction to be overcome as well as the 
force of gravity. Slightly harder work is done at the ex- 
pense of the muscular energies of the operator and this is 



213 



FRICTTONAL ELECTRICITY, 



the real origin of the energy stored up in the separate 
charges/^ 

Experiment 28. — Insulate a metal globe and provide it with two 
closely fitting hemispherical shells that have insulating handles. 
Electrify the globe ; bring it near the electroscope to be sure that it is 
electrified. Place the hemispheres upon the globe. Remove them 
quickly, being careful that their edges do not touch the sphere 
after the first separation. (Fig. 138.) Bring first one shell and then 
the other near the electroscope ; they are electrified. Bring the globe 
itself near the electroscope. It is no longer electrified. Delicate 
manipulation is needed to make the experiment successful. You 
will fail, perhaps, more times than you succeed. But when the 
experiment is successful, it is instructive. The apparatus is called 
Biot's hemispheres. 




Fig. 139. 

Experiment 29.— By means of a few sparks from the electro- 
phorus, charge an insulated hollow sphere, having an orifice in the 



FRICTIONAL ELECTRICITY, 



213 



top. Bring a.proof plaue (made by fastening a disc of gilt paper to 
a long, thin insulating handle) into contact with the outer surface 
of the sphere. The proof-plane is charged by the sphere, as may be 
shown by bringing it near an electroscope. Discharge the proof - 
plane and bring it into contact with the inner surface of the sphere. 
Remove it carefully without allowing it to touch the sides of the 
orifice. Bring it to the electroscope. It is not charged. (Fig. 139.) 
An empty tin fruit can supported on a clean, dry, glass tumbler will 
answer for the experiment. 



Experiment 30. — Make a conical bag 
of linen, supported, as shown in Fig. 
140, by an insulated metal hoop five or 
six inches in diameter. Charge the bag 
with the electrophorus. A long silk 
thread extending each way from the 
apex of the cone will enable you to turn 
the bag inside out without discharging 
it. Test the inside and outside of the 
bag, using the proof -plane described 
above. Turn the bag and repeat the 
test. Whichever surface of the linen is 
external, no electricity can be found upon 
the. inside of the bag. Nothing can be 
more conclusive than this. 




Fig. 140. 



Experiment 31. — Vary the experiment by the use of a hat sur^- 
pended by silk threads. Notice that the greatest charge can be 
obtained from the edges ; less from the curved or flat surface ; none 
from the inside. 



341. A Charge Resides on the Surface.— 

Many experiments have been made showing that when a 
conductor is electrified, the electricity passes to the 
surface and escapes if the body he not insulated. 
A bomb-shell and a cannon ball of equal diameter will 
receive equal quantities of electricity from the same source. 
The hollow conductors commonly used in experiments with 
static electricity are as seryiceable as if they were solid. 
A wooden prime conductor coated with gold-leaf is as 



214 FRICTIONAL ELECTRICITY, 

efficient as if it were made of solid gold. Experiment is 
unable to find any difference in this respect between a 
solid sphere of metal and the thinnest soap-bubble of the 
same diameter. 

{a.) This does not apply to an electric current. A hoHow wire 
will not conduct electricity as well as a solid wire of the same 
diameter. Electricity may be drmcn to the inside of a hollow con- 
ductor by placing there an electrified, insulated body. 

(6.) The linen bag of Experiment 30 was devised by Michael 
Faraday, but his most striking experiment was made with a wooden 
cage, measuring 12 feet each way, covered with tin-foil, insulated 
and charged by a powerful electric machine. He carried his most 
delicate electroscopes into this cage. Large sparks and brushes 
were darting oflP from every part of the outer surface, but the phil- 
osopher and his sensitive instruments within the cage failed to 
detect the least electric influence. 

Experiment 32. — Place a carrot horizontally upon an insulating 
support. Into one end of the carrot, stick a sewing-needle. Bring 
the electrified glass rod near the point of the needle without touching 
it. The — . electricity of the carrot quietly escapes from the point 
to the rod and the carrot is charged with the + electricity that 
remains. 

343. Density. — Experiments show that when a spher- 
ical conductor is charged^ the electricity is evenly dis- 
tributed over the surface, provided no other electrified 
body be near to affect the distribution by induction. The 
electric density (or number of electrical units per unit of 
area) is the same at every point. Experiments on an 
elongated cylinder, like the prime conductor of the elec- 
tric machine, show that the density is greater at the ends. 
On an egg-shaped conductor, like that shown in Fig. 141, 
the density is greatest at the smaller end. In general, 
the electric density is very great at any pointed 
part of a charged conductor. 



FRICTION A L ELECTRICITY, 



215 



This density at a point may become so great that the 
electricity will escape rapidly and quietly, the air particles 




Fig. 141. 

quickly carrying off the charge by convection. This 
explains the effect of pointed conductors, which plays so 
important a part in the action of electric machines. This 
property will be illustrated in several of the experiments 
of § 371. It is fundamental to the quiet action of light- 
ning rods. 

343. Electric Machines. — Machines have been 
made for developing larger supplies of electricity more 
easily than can be done with a rod of glass or sealing-wax 
or with the electrophorus. Each of them consists of one 
part for producing the electricity and another part for 
collecting it. 

344. The Plate Electric Machine.— This in- 
strument is represented in Fig. 142. It consists of an in- 
sulator (or electric), a rubber, a negative and a positive or 
prime conductor. The electric is a glass (or ebonite) plate, 
A, generally one, two or three feet in diameter. This plate 
has an axis, B, and handle, C, and is supported upon two 
upright columns. The rubber, D^ is made of two cush- 



216 FRICTIONAL ELECTRICITY. 

ions of silk or leather, covered with amalgam (see § 302, a). 
They press upon the sides of the plate and are supported 



Fig. T42. 

from the negative conductor, with which they are in 
electric connection. The negative conductor, N, is sup- 
ported upon an insulating column and, when only posi- 
tive electricity is desired, is placed in electrical connection 
with the earth by means of a chain or wire, W, The 
prime conductor, P, is insulated. One end of the prime 
conductor terminates in two arms, P, which extend one 
on either side of the plate. These arms, being studded 
with points projecting tow^ard the plate, are called comhs. 
The teeth of the combs do not quite touch the plate. A 
silk bag, S, is often supported so as to enclose the lower 
part of the plate. All parts of the instrument except the 
teeth of the combs are carefully rounded and polished, 
sharp points and edges being avoided to prevent the es- 
cape of electricity as already explained. This avoiding of 
points and edges is to be regarded in all apparatus for use 
with electricity of high potential. 



FBICTIONAL ELECTRICITY, 2lT 

(a.) The pupil may make a plate machine without much expense. 
A glazier will cut for him a disc of plate glass, possibly from a 
fragment on hand. The edges of this disc may be rounded on a wet 
grindstone. A hole may be bored in the middle with a round file 
kept moistened with a solution of camphor in turpentine. The con- 
ductors, N and P, may be made of wood covered with gold-foil or 
Datch leaf and supported on pieces of stout glass tubing. The 
prime conductor may well have two such supports. The arms may 
consist of two stout wires thrust into the end of a prime conductor, 
their free ends being provided with knobs of lead or other metal. 
The combs may be made by soldering pin points to one side of each 
arm. See that the gold-foil makes actual contact with the metal 
arms. See that all metal parts except the pin points are polished 
smooth. The columns that support the plate may be made of sea- 
soned wood. The part of the handle to which the hand is applied 
may be made of glass or insulated by covering it with rubber 
tubing. 

345. Operation of the Plate Machine.— The 

plate is turned by the handle. Electric separation is pro- 
duced by the friction of the rubbers. The + electricity of 
the rubber and negative conductor passes to the plate; the 
— electricity of the plate passes to the rubber and negative 
conductor. The part of the plate thus positively charged 
passes to the combs of the prime conductor. The + of 
the plate acts inductively upon the prime conductor, polar- 
izes it, repels the + and attracts the — electricities. Some 
of the — electricity thus attracted streams from the points 
of the combs against the glass, while some of the + elec- 
tricity of the glass escapes to the prime conductor. This 
neutralizes that part of the plate, or restores its electric 
equilibrium, and leaves the prime conductor positively 
charged. As each successive part of the plate passes the 
rubber, it gives off — electricity and takes an equal 
amount of + ; as it passes between the combs it gives off 
its + electricity and takes an equal amount of — . The 



^18 FRICTIONAL ELECTRICITY, 

rubber and negative conductor are kept in equilibrium by 
means of their connection with the earth, '^the common 
reservoir/^ As the plate revolves, the lower part, passing 
from iV^to P, is positively charged; the upper part, pass- 
ing from P to iV, is neutralized. If negative electricity 
be desired, the ground connection is changed from N to 
P and the charge taken from N, 

346. The Dielectric Machine. — This instru- 
ment is represented in Fig. 143. Two plates of vulcanite 
(ebonite), A and jB, overlap each other without touching 
and revolve in opposite directions. The upper plate is 
made to revolve much more rapidly than the lower by 
means of the pulleys shown at the right of the figure. 
The prime conductor and the axes of the two plates are 
carried by two insulating pillars. From the prime con- 
ductor, a comb is presented to the upper part of the upper 
plate. Another comb is presented to that part of A which 
is overlapped by the upper part of B, This comb is con- 
nected by a universal joint at e with a discharging rod and 
ball, which may be brought near the end of the prime 
conductor or turned away from it. The rubbers and the 
lower comb are to be in electrical communication with 
the earth. The general arrangement is clearly set forth 
in the figure. 

347. Operation of the Dielectric Machine. — 

The plate, B, is turned directly by the handle and the 
plate. Ay indirectly by the aid of the pulley. The plate, 
5, is negatively electrified by friction with the rubber 
and thus acts by induction upon the lower part of A^ 
which is thus polarized. The + of this part of A is 



FRICTIONAL ELECTRICITY. 



219 



bound by the attraction of the — of B, while the — of 
A is repelled, escapes by the lower comb and is replaced 
by + from the earth 
through the lower 
comb and its ground 
connection. This part 
of A, thus positively 
charged, is soon re- 
moved from the induc- 
ing body and the + 
charge, hound by B, is 
set free. It then 
comes to the upper 
comb, polarizes it and 
the prime conductor 
and exchanges some of 
its own + for an equal 
amount of — from the 
prime conductor. This 
neutralizes that part of the upper plate and leaves the 
prime conductor positively charged. As each successive 
part of A passes the lower comb, it gives off — electricity 
and takes an equal amount of + ; as it passes the upper 
comb, it. gives off + electricity and receives an equal 
amount of — . The charge of B is continually main- 
tained by friction with the rubber. When the discharging 
rod and ball are brought near the prime conductor, as 
shown in the figure, a rapid succession of sparks is pro- 
duced, owing to the recombination of the separated elec- 
tricities. If another body is to be charged from the prime 
conductor, the ball and rod may be turned aside. The 
efficiency of this machine is greater than that of the plate 




Fig. 143. 



220 



FRICTTONAL ELECTRICITY. 



or cylinder machine. It is less affected by atmospheric 
moisture and is more compact^ but the vulcanite plates 
seem to deteriorate with use. They should be washed 
occasionally with ammonia water and rubbed with paraf- 
fin oil. Machines of similar construction, but having 
glass plates, are made. 

348. The Holtz Electric Machine.— This in- 
strument is represented in Fig. 144. It contains two thin^ 

circular plates of 
glass^ the larger 
of which is held 
fast by two fixed 
pillars. The 
smaller plate re- 
volves rapidly 
very near it. 
There are two 
holes in the fixed 
Fig. 144. plate near the 

extremities of its horizontal diameter. To the sides of 
these openings are fastened paper bands called armatures. 
The armatures point in a direction opposite to that in 
which the revolving plate moves. Opposite these armatures 
and separated from them by the revolving plate, are two 
metallic combs, connected respectively with the two knobs 
and Leyden jars shown in the front of the picture. One of 
these knobs is carried by a sliding rod so that their distance 
apart is easily adjusted. When this machine works well, it 
gives results superior to either of those previously mentioned. 
It is, however, peculiarly subject to atmospheric conditions 
and is generally considered extremely capricious. 




FRICTIONAL ELECTRICITY. 'Z'Zl 

349. Action of the Holtz Machine. — To un- 
derstand the action of this machine requires careful atten- 
tion. The knobs are placed in contact and a small initial 
charge is given to one of the armatures by some charged 
body, as a piece of vulcanite or a glass rod. The handle 
is then turned, the effort necessary to keep up the motion 
increasing rapidly. The knobs are then separated and a 
series of discharges takes place between them. 

{a.) Suppose a small + charge to be imparted at the outset to the 
right armature. This charge acts inductively across the revolving 
plate upon the metallic comb, repels + electricity through it and 
leaves the points negatively electrified. They discharge negatively 
electrified air upon the front surface of the movable plate ; the re- 
pelled -f charge passes through the brass rods and balls and is dis- 
charged through the left comb upon the front side of the movable 
disc. Here it acts inductively upon the paper armature, causing 
that part of it which is opposite itself to be negatively charged and 
repelling a + charge into its farthest part, viz., into the armature. 
This, being bluntly pointed, slowly discharges a + charge upon 
the hack of the movable plate. When the plate is turned round, 
this -F charge on the back comes over from the left to the right side 
and, when it gets opposite the comb, increases the inductive effect 
of the already existing + charge on the armature and, therefore, 
repels more electricity through the brass rods and knobs into the 
left comb. Meantime the — chal'ge, which we saw had been in- 
duced in the left armature, has in turn acted on the left comb, caus- 
ing a + charge to be discharged by the points upon the front of the 
plate and, drawing electricity through the brass rods and knobs, 
has made the right comb still more highly — , increasing the dis- 
charge of negatively electrified air upon the front of the plate, 
neutralizing the -f charge which is being conveyed over from the 
left. These actions result in causing the top half of the moving 
disc to be positively electrified on both sides and the bottom half of 
the disc to be negatively electrified. The charges on the front 
serve, as they are carried round, to neutralize the electricities let off 
by the points of the combs while the charges on the back, induced 
respectively in the neighborhood of each of the armatures, serve, 
when the rotation of the plate conveys them round, to increase the 
inductive influence of the charge on the other armature. Hence, a 



322 FRICTTONAL ELECTRICITY. 

very small initial charge is speedily raised to a maximum, the limit 
being reached when the electrification of the armatures is so great 
that the loss of electricity at their surface equals the gain by con- 
vection and induction. 

]!^ote. — Other forms of electric machines are made. One of the 
latest of these, known as the Toepler-Holtz, is very compact and 
efficient and remarkably free from the limitations of atmospheric 
conditions. It may be described as a continuously acting electro- 
phorus (§ 227). A very good one may be bought for $25 or more. 
One should be provided for the school in some way if possible. Any 
electrical machine should be free from dust and perfectly dry when 
used. It should be warmer than the atmosphere of the room, that 
it may not condense moisture from the surrounding air. The drier 
the atmosphere, the better will be the action of the machine. 

Exercises. 

1. How can you show that there are two opposite kinds of elec- 
tricity ? 

2. How would you test the kind of electricity of an electrified 
body? 

3. Quickly pass a rubber comb through the hair and determine 
whether the electricity of the comb is positive or negative. 

4. Why do we regard the two electric charges produced simul- 
taneously by rubbing together two bodies as being of opposite 
kinds ? 

5. Why is it desirable that a glass rod used for electrification be 
warmer than the atmosphere of the room where it is used? 

6. Electrify one insulated egg-shell conductor (§ 332, 6). Bring it 
near a second conductor but not in contact with it. Touch the 
second egg-shell with the finger, {a.) Experimentally, determine 
whether the second egg-shell is electrified or not. (6.) If you find 
that it is, what word explains the method of charging? (c.) If the 
second egg-shell is charged, will its potential and the potential of the 
first be of the same or of opposite signs ? 

7. {a.) In § 323, h, it is directed that an electrified body be brought 
** near" the knob of the gold-leaf electroscope. Why not touch the 
knob with the charged body? (&.) Why do not the gold leaves 
diverge immediately after touching the knob with the finger as there 
directed? (c.) If the electrified body being tested had a + charge, 
is the charge of the gold leaves -F or — ? Explain. 

8. (a.^ What is a proof-plane? (b.) An electroscope? (c.) Describe 
one kind of electroscope, {d.) Another kind. 



FRICTIONAL ELECTRICITY, 22o 

9. (a) Define electrics, conductors and insulators, (b.) Explain 
electric induction. 

10. (a,) IF a metal globe suspended by a silk cord be brought near 
the prime conductor of an electric machine in action, feeble sparks 
will be produced. Explain, [b.) If the globe be held in the hand, 
stronger sparks will be produced. Explain. 

11. Twist some tissue paper into a loose roll about six inches long. 
Stick a pin through the middle of the roll into a vertical support. 
Present an electrified rod to one end of the roll and thus cause the 
roll to turn about the pin as an axis. Give this piece of scientific 
apparatus an appropriate name. 

12. (a.) Prepare two wire stirrups, A and B, like those shown in 
Fig. 131 and suspend them by threads. Electrify two glass rods 
by rubbing them with silk and place them in the stirrups. Bring 
A near B. Notice the repulsion, (b.) Repeat the experiment with 
two sticks of sealing-wax that have been electrified by rubbing with 
flannel. Notice the repulsion, (c.) Place an electrified glass rod in A 
and an electrified stick of sealing-wax in B. Notice the attraction. 
Give the law illustrated by these experiments. 

13. Two small balls are charged respectively with -f 24 and — 8 
units of electricity. With what force will they attract one another 
when placed at a distance of 4 centimeters from one another ? 

Ans. 12 dynes. 

14. If these two balls are then made to touch for an instant and 
then put back in their former positions, with what force will they 
act on each other ? Ans. Repulsion of 1 dyne. 



224 



FRICTIONAL ELECTRICITY. 



Experiment 33. — Hang a negatively charged pith ball inside a 
dry glass bottle. Bring an electrified glass rod to the outer side of 
the bottle. The pith ball will rash to the side of the bottle nearest 
the rod because of the attraction between the opposite electricities. 

Experiment 34. — Paste a piece of tin-foil, two or three inches 
square, on the middle of each face of a pane of glass. Hold a finder 
on one of the metallic coats while the other coat is held, for a short 
time, iu contact with the prime conductor of an electric machine in 
operation. Remove the pane and place it on edge without touching 
both coats at the same time. Although both coats are oppositely 
charged (§ 334), they may be touched in succession without any 
shock. When both are touched at the same time, the shock is 
greater than would have been received from the prime conductor by 
which this condense/r was charged. 

350. Condensation of Electricity. — Two 

suspended pith balls oppositely charged attract one 
another across the intervening air. They attract mu- 
tually even when a plate of glass is held between them 

although neither the balls 
^' nor their electric charges can 

pass through the glass. In 
the case of the pane of 
glass with its two tin-foil 
coats, or in the similar case 
of two metallic plates, A 
and B^ separated by a layer 
of dry air or other non- 
conductor, (7, as shown in 
Fig. 14e5, the two charges 
are '^bound,^^ each by the 
attraction of its opposite 
on the other side of the 
pane. It is found that two such coats may be charged 
much more strongly than either one could be if the oppo- 
site coat were wanting. If a third plate like B, but hav- 




Fig. 145. 



FRICTJONAL ELECTRICITY, 



225 



ing no opposite plate like Ay be connected with 5 by a 
copper wire and the middle of the wire brought into con- 
tact with the prime conductor, nearly the whole charge 
will go to B and very little to the third plate. The ca- 
pacity of a charged conductor is greatly increased 
by bringing it near a second charged conductor 
oppositely charged. Its capacity being thus increased, 
a greater quantity of electricity must be put into it to 
raise it to as high a potential. Such a method of increas- 
ing the quantity of electricity that a conductor may re- 
ceive without raising its potential is called the condensa- 
tion or accumulation of electricity. 

351. Electric Condensers. — An apparatus for 
collecting a large quantity of electricity at a moderate po- 
tential, as just described, is called an electric condenser. 




(a.) Let A and B, Fig. 146, 
represent two insulated metallic 
plates about six inches in diam- 
eter, separated by (7, a plate of 
glass somewhat larger. Let each 
metallic plate have an electric 
pendulum, a and 5. Remove A 
and connect B with the conductor 
of the electric machine, by means 
of the wire, x. The divergence of 
h shows the presence of free electricity. Connect A with the 
ground by the wire, ?/, and place it in position as represented. By 
the inductive influence of B, the - electricity of A is drawn to the 
surface, n, while the + escapes by y. But this — electricity at n 
attracts the + of B largely to the surface m and holds it there as 
hound electricity, thus increasing the electrical density at that sur- 
face. This change is shown by less divergence of h. Consequently, 
B can receive more electricity from the machine, which will, in 
turn, attract more — electricity to n. This further supply will, in 



FRICTTONAL ELECTltlClTY. 




Fig. 147. 



turn, bind more of the + electricity of B at m. In this way, a large 
quantity of + electricity may be accumulated at m and a large 

quantity of -— at n. 
This accumulation may 
thus go on until the 
potential at the sur- 
face, p, is equal to that 
of the machine, as it 
was when A was ab- 
sent. Interrupting 
communication by x 
and y, both plates are 
charged. The vertical 
pendulum, a, shows no 
free electricity, the 
electricity of A being 
all bound at n ; the 
pendulum at 5 shows 
some free electricity, 
although the greater part of the electricity of B is bound at m. Re- 
move A and B from each other and the bound electricity of each is 
set free and both a and h fly out as the discs are separated. The 
pith balls thus seem to indicate that the discs are more highly elec- 
trified when they are thus separated, but no additional charge has 
been given to either A or B. The fact is that while B was near Ay 
the capacity of B was largely increased. On moving it away from A, 
its capacity was diminished and the same quantity of electricity elec- 
trified it to a higher potential than before. The presence of an earth 
connected plate near an insulated conductor largely increases the elec- 
tive capacity of the latter^ enabling it to condense electricity upon the 
furface nearest the opposing plate, at lohich surface the electrical den- 
sity becomes very great. 

(b.) If A and B are pushed up close to O, the decrease of distance 
will work an increase of the inductive action and a still larger quan- 
tity may be accumulated in the plates. Thus, the capacity of a con- 
denser depends, in part, upon the nearness of the plates to each other. 



353. Dielectrics and Specific Inductive 
Capacity. — Substances that permit inductive electric 
influences to act across or through them as just described 
are called dielectrics. All dielectrics are insulators, but 



FRICTIONAL ELECTRICITY. 



327 



equally good insulators are not always equally good dielec- 
trics. Glass is a better dielectric than ebonite and ebonite 
is better than air. The capacity of a condenser is greater 
when the dielectric is glass than it is when the dielectric 
is air. The ratio of the capacity in the former case 
to the capacity in the latter case is called the 
specific inductive capacity (or specific indiictivity ) 
of glass. Air (at 0° C. and 760 mm,) is taken as the 
standard, its specific inductive capacity being unity. 

(a.) The old idea that electric induction is " action at a distance" 
is wholly disproved by the fact that different substances have dif- 
ferent specific inductive capacities, for it is evident that the dielec- 
tric itself is concerned in the process. Otherwise, all media would 
allow induction to take place across them with equal facility. 

(&.) The specific inductivity (sometimes called dielectric capacity) 
assigned to various substances by different observers varies widely. 
Gordon gives the following results : 



Air 1.00 

Paraflan (solid).1.9936 
India rubber. .2.22 



Ebonite 2.284 

Gutta percha..2.462 

Sulphur 2.58 

Schiller gives the specific inductivity of white mirror glass as 5.88 
to 6.34 



Shellac 2.74 

Glass, from. ...3.013 
•' to 3.258 



353. The Leyden Jar. — The most com- 
mon and, for many purposes, the most con- 
venient form of condenser is the Leyden ja|:. 
This consists of a glass jar, coated within and 
without for about two-thirds its height with 
tin-foil, and a metallic rod, communicating 
by means of a small chain with the inner coat 
and terminating above in a knob. The upper 
part of the jar and the cork which closes the 
mouth of the jar and supports the rod are generally coated 
with sealing-wax or shellac varnish to lessen the deposition 




di'!i;!iii!iiiiii 
Fig. 148. 



338 FRICTION A L ELECTRICITY. 

of moisture from the air. The inner coat represents the 
collecting plate, jB; the glass jar, the insulating plate, (7; 
the outer coat, the condensing plate, A, of Fig. 146. 

{a.) Select a candy or fruit jar of greenish glass ; paste tin-foil 
within and without, as above described, using flour paste ; thrust a 
wire through a dry cork ; bend the wire so that, when the cork is in 
its place, the wire shall touch the tin-foil on the inside of the bottle 
without tearing it ; solder the upper end of the wire to a smooth 
button or thrust it into a lead bullet ; charge your Leyden jar with 
a few sparks from the electrophorus and take a shock. 

354. Charging the Leyden Jar. — To charge 
the jar, hold it in the hand, as shown in Fig. 149, and bring 
the knob near the prime conductor of an electrical machine 
that is in action or into contact with it. 

{a.) The + charge thus developed on the inner coat acts in- 
ductively through the glass, repelling the + electricity which escapes 
through the hand to the earth and binding its — electricity to the 
surface in contact with the glass. This '* bound " negative elec- 
tricity of the outer coat, in turn, binds the positive of the inner coat, 
which then may receive a further charge and so on. The inner 
coat will receive a much greater quantity of electricity than it pos- 
sibly could were it not for the attraction of its opposite on the outer 
coat. If, instead of holding the outer coat in the hand, the jar be 
supported upon a pane of glass so that the repelled electricity of the 
outer coat cannot escape, the jar cannot be very intensely charged. 




Fig. 149. 

(6.) Thus we see again that the capacity of a conductor is greatly 
increased when it is placed near a conductor charged with the oppo- 




FRICTIONAL ELECTRICITY. 229 

site kind of electricity. Its capacity being increased, it can receive a 
greater quantity of electricity without any increase of potential. Of 
course, the potential of the charged jar cannot exceed that of the 
prime conductor or other charging body. 

355. Discharging the Leydeii Jar.— If the jar 

be of good glass, dry and free from dust, it will retain its 
charge for hours. But if a path be provided by which the 
opposite and mutually attracting electricities can flow 
together, they will do so and 
the jar will be instantaneously 
and almost completely dis- 
charged. The jar might be 
discharged by touching the 
knob with the finger, the sep- 
arated electricities comin^r to- t- 

° Fig. 150. 

gether through the person of 

the experimenter and the earth. In this case, the experi- 
menter will feel a "shock.^^ If the charge be intense, the 
shock will be painful or even dangerous. It is better to 
use a " discharger," two forms of which are represented in 
Fig. 150. This consists of two metal arms hinged to- 
gether, bearing knobs at their free ends and carried by 
insulating handles. The outer coat of the jar should be 
touched first. Why ? 

(61^.) A good discharger may be made by passing a piece of stout, 
copper wire, about a foot long, through a piece of rubber tubing and 
providing a metal knob for each end of the wire. The flexibility of 
the wire avoids the necessity for a hinged joint. 

356. The Residual Charge. — If a Leyden jar be 
charged, discharged and left for a little time to itself, it 
will be found that a small, second spark can be obtained. 



230 



FRICTIONAL ELECTRICITY. 



There is a residual charge which seems to have 
soahed into the glass. The return of the residual charge 
is hastened by tapping the jar. The 
amount of the residual charge varies 
with the time that the jar has been 
left charged ; it also depends on the 
kind of the glass of which the jar is 
made. (See Appendix J.) 

357. The Ley den Jar with 
Movable Coats.— this piece of appara- 
tus is represented by Fig. 151. The upper 
part of the glass jar, B, is coated with shel- 
lac varnish. The three parts being placed 
together in proper order, B within J. and (7 
within ^, the jar is charged in the usual man- 
ner. The inner coat, C, is then removed 
with a glass rod and touched with the hand 
to discharge it fully. B is then lifted out 
from A and the outer coat fully discharged. 
The three parts are then put together again 
and found to be able to give nearly as strong 
a spark as at first. This seems to indicate 
that the charge rests upon the surfaces of the glass rather than upon 
the surfaces of the coats. If, when the charged jar is in pieces, the 
thumb be placed on the outer surface of the glass and the forefinger 
of the same hand on the inner surface, a very slight shock is per- 
ceptible. The oppositely charged glass molecules that come into 
actual contact with thumb and finger respectively are discharged. 
By changing the position of the thumb and finger, successive little 
shocks may be felt as successive portions of the inner and outer sur- 
faces of the glass are discharged. The inner coat furnishes a means 
for the simultaneous discharge of the inner layer of glass molecules ; 
the outer coat does the same for the outer layer of glass molecules. 
Thus all or nearly all of the electrified glass molecules may be dis- 
charged simultaneously instead of successively. 




Fig. 151. 



358. The Leyden Battery.— The effect that may 
be produced with a Leyden jar or other condenser depends 



FRICTIONAL ELECTRICITY. 



231 



upon the size of the coats, the thinness and the inductive 
capacity of the glass. But a large jar is expensive and 
requires great care ; thin glass is liable to perforation by 




the condensed and strongly attracting electricities of its 
two coats. To obviate both of these diflficulties, a collection 
of jars is used. When their outer coats are in electric 
communication, which may be secured by placing them 
in a tray the bottom of which is covered with tin-foil, and 
their inner coats are connected by wires or metal strips 
passing from rod to rod, or from knob to knob, the ap- 
paratus is called a Leyden or electric battery. ^^Tough- 
ened glass" is less easily pierced than ordinary glass. 
Hence, Leyden jars made of it may be made thinner and, 
consequently, will hold a greater charge than otherwise. 
The battery is charged and discharged in the same way as 
a single jar. Great care is needed, for if the discharge 



232 FRICTIONAL ELECTRICITY. 

were to take place through the human body the result 
would be serious and possibly fatal. The ^^ universal 
discharger/^ as employed with the Leyden battery^ is 
shown at AG i\\ Fig. 152. (See Exp. 56.) 

{a.) The horizontal rods of the universal discharger may be sup- 
ported by passing them through corks in the mouths of two bottles. 
When a table is wanted for the support of bodies to be operated 
upon by the discharge, it may be made by placing a small plate of 
glass upon the open mouth of a bottle of the same height as those 
that carry the rods and placing the third bottle between the other 
two. 

359. The Farad. — The farad is the capacity of a 
condenser that will be raised to a potential of one yolt by 
a charge of one coulomb of electricity (§§ 382, 387). Such 
a condenser would be too large to be constructed. T]%6 
micro-farad (—0.000001 farad) is, therefore^ chosen as 
the -practical unit of electrical capacity. The ca- 
pacity of three miles of an Atlantic cable is about one 
micro-farad. A micro-farad condenser contains about 
3,600 square inches of tin-foil. A farad equals 10"^ of an 
absolute unit of capacity (§ 330). See Appendix M (5). 

(a,) A coulomb in a farad gives a volt. 

Coulombs 



Farads = 



Volts. 



360. Submarine Cable Condensers. — An 

ocean cable forms a condenser, the water forming the 
outer coating ; the conducting wire, the inner coating; 
while the insulating layers of gutta-percha correspond 
to the glass of the Leyden jar. When, for example, one 
end of a submerged cable is connected to the + pole of 
a powerful battery, + electricity flows into it. Before 
any signal can be receiyed at the other end, enough elec- 



FRICTIONAL ELECTRICITY, 



233 



tricity must flow in to charge the cable to a considerable 
potential, an operation which may, in the case of long 
cables, require some seconds. It is a serious obstacle to 
signalling with speed through the Atlantic cables. 

{a.) Imagine a mile of insulated cable wire to be coiled up in a tub 
Df water (Fig. 153), one end, N, being insulated. The other end is 
joined up through a long ^^.^ 

coil galvanometer, G, to j g\ 

the + pole of a large bat- ^ ^ // rS\ ^ — ^ 

tery, whose — pole is 
joined by a wire to the 
water in the tub. As 
soon as this is done, the 
needle of the galvanom- 
eter will show a violent 
deflection, + electricity rushing through it into the interior of the 
cable and a — charge being accumulated on the outside of it where 
the water touches the gutta-percha. The flow will go on, though 
diminishing, until the cable is fully charged, taking, perhaps, an 
hour. Now remove the battery and close the circuit. The charge 
in the cable will rush out through the galvanometer, which will 
show an opposite deflection. The charge will continue *'to soak 
out " for a long time. 




Fig. 153. 



361. Modes of Discharge. — An electrified con- 
ductor may be discharged in at least three ways, viz,, by 
the disruptive discharge, by the convective discharge 
and by the conductive discharge. The discharge in any 
of these ways is accompanied by a transformation of en- 
ergy. Sound, light, heat, chemical action and other phe- 
nomena are produced. 

Experiment 35. — Present a knuckle of the hand or a metal knob 
to the prime conductor of an electric machine and "draw sparks" 
therefrom. (See Fig. 169.) For short distances, the spark is straight. 
If the distance be made somewhat greater, the spark takes a sinuous 
and forked form as though floating dust particles served as stepping- 
stones and rendered a crooked path the easiest. If the charge be 



234 



FRICTIONAL ELECTRICITY. 



very powerful, the spark will take tlie zigzag form so familiar in the 
lightning-stroke. When the machine is vigorously worked in the 
dark, the apparently continuous discharge into 
the air produces a luminous appearance at the 
ends of the conductor. This appearance, known 
as a brush, may be improved by holding a large, 
smooth, metal globe at a distance a little too great 
for the passage of a spark. When the discharge 
takes place from the rounded end of a wire ex- 
tending from the conductor, a quiet, phosphor- 
escent glow, as shown in Fig. 154, will often appear at and near the 
end of the wire. 




Fig. 154. 



362. The Disruptive Discharge. — A discharge 
of electricity taking place suddenly through a non-con- 
ductor is called a disruptive discharge, e.g., the spark and 
brush drawn from an electric machine in action. The 
glow is either a continuous discharge or one of exceed- 
ingly small period. Perhaps, it is a high order of con- 
vectiye discharge. 

Experiment 36. — Attach a pointed wire to the prime conductor 
of the electric machine. The flame of a candle held near will be 
blown away, as shown in 
Fig. 155. If the candle be 
placed upon the prime 
conductor and a pointed 
conductor be held in the 
hand near the candle, the 
flame will still be blown 
away. 

363. The Con- 
vective Dis- 
charge.^ — When elec- 
tricity of high poten- 
tial accumulates with 
so great a density as to electrify the neighboring particles 
of air which^ driyen by electric repulsion, fly off carrying 




Fig. 155. 



FRlCTtONAL ELECTRICITY. 235 

part of the charge with them, we have what is called the 
convective discharge. Such discharges are best mani- 
fested in gases at low pressure, in tubes exhausted by an 
air-pump. (Exp. 70.) 

364. The Conductive Discharge.— The flow 
of a continuous current of electricity constitutes the con- 
ductive discharge. When electricity flows through a wire 
from the prime conductor of an electric machine to the 
rubbers or from the positive pole of a voltaic cell or bat- 
tery to the negative, we have a conductive discharge. It 
will be considered in the section especially devoted to 
voltaic electricity. 

365. Atmospheric Electricity.— The phenom- 
ena of atmospheric electricity are of three kinds : 

1. A continual slight electrification of the air, best ob- 

served in fair weather. 

2. The familiar phenomena of thunder storms. 

3. The Aurora Borealis. 

366. Tlie First Kind. — During fair weather, the 
air above the surface of the earth is usually electrified posi- 
tively, a negative electrification being extremely rare. In 
stormy weather, it is more often — than + and frequently 
changes from one kind to the other several times in an 
hour. The higher up we go to observe the usual + elec- 
tricity of the air, the higher its potential is found to be. 
The evaporation of water by the sun's heat and the fric- 
tion of moving masses of air probably contribute to the 
presence of atmospheric electricity. 

367. Thunder Storms.— We have already seen 
(§ 341) that a solid conductor can not be charged through- 



236 FRICTTOKAL ELECTRICITY, 

out its substance^ the charge residing upon the surface. 
The same is true of liquids^ but aeriform bodies may be 
charged bodily, the individual molecules being so much 
more widely separated. Dry air being a poor conductor, 
the air particles discharge their electricity into each 
other slowly and with difficulty. The electricity thus 
prevented from accumulating has a low potential and, 
hence, gives few manifestations of its presence. The 
minute particles of water floating in the air being better 
conductors than the air itself become more highly charged. 
As they fall and unite, the potential of their charges in- 
creases. 

{a,) "Suppose eight small drops to join into one. That one will 
have eight times the quantity of electricity distributed over the sur- 
face of a single sphere of twice the radius and, therefore, of twice 
the capacity (for the electrical capacities of spheres are proportional 
to their radii) of the original drops. '^ The capacity being thus in- 
creased only tv/o fold while the quantity is increased eight fold, the 
potential becomes four times as great. Thus the potential of a 
cloud may rise by the union of electrified drops. 

368. Lightning.— When an electrified cloud floats 
over the earth, separated from it by a layer of insulating 
air, the inductive influence of the cloud renders the ground 
beneath oppositely electrified. Then the cloud, ground 
and insulating air correspond respectively to the inner 
and outer coatings and the insulating glass of a Leyden 
jar. As the charge of a Leyden jar may be made so in- 
tense that the mutual attraction of the separated elec- 
tricities will result in their rushing together and thus 
piercing the jar (§ 358), so the charge of a cloud may be- 
come sufficiently intense to overcome the resistance of the 
air and a lightning stroke ensues. Two clouds charged 



FRICTIONAL ELECTRICITY, 237 

with opposite electricities may float near each other. 
Then they, with the intervening air, may be looked upon 
as constituting a huge Leyden jar. Thus, we may see 
the lightning leaping from cloud to earth, or from cloud 
to cloud. Such electric sparks are sometimes more than 
a mile in length, showing a difference of potential greater 
than that of 3,000,000 Daniell's cells. The duration of 
the spark or flash is not more than 0.00001 of a second. 
The danger from any lightning stroke has passed when we 
hear the crash. The identity of lightning with electricity, 
though long suspected, was first proved by Franklin's 
famous kite experiment. 

Experiment 37. — Bring the point of a knife-blade near the con- 
ductor of an electric machine in operation and notice the instant cessa- 
tion of sparks. The quiet passage of electricity from the earth 
neutralizes the charge of the conductor and restores the electric 
equilibrium. In the same way, a lightning-rod tends to restore the 
electric equilibrium of the cloud and prevent the dangerous dis- 
charge. 

369. Lightning-Rods. — The value of lightning- 
rods depends upon the tendency of electricity to follow 
the best conductor and upon the effect of pointed con- 
ductors upon electrical density (§ 342). The lightning- 
rod should, therefore, be made of a good conductor ; 
copper is better than iron. It should terminate above in 
one or more points, tipped with some substance that may 
be corroded or fused only with extreme difficulty. Plati- 
num and iridium are metals that satisfy these conditions 
very well. The rod should extend above the highest point 
of the building in order to offer the electricity the easiest 
path to the ground. It is important to have each pro- 
jecting part of the building, as chimneys, towers and 



338 FRICTIONAL ELECTRICITY. 

gables, protected by a separate rod. All metal work about 
the roof or chimneys should be connected with the rod. 
The rod should afford an unbroken connection; the joints, 
if there be any, should be carefully made. The rod 
should terminate below in water, or in earth that is ahvays 
moist. It is well to connect it with underground water- 
pipes when possible or with a large metal plate. Personal 
attention should be given to this matter when the rod is 
put up as, being under ground and out of sights this part 
of the rod is not easily inspected subsequently. A rod 
having a blunted tip, a broken joint or terminating 
in dry earth is more dangerous than no rod at all. 
Lightning-rod insulators are undesirable. 

(a.) The greatest value of a lightninpr-rod is due to its quiet work 
in tlie prevention of the lightning stroke. For this quiet but very 
valuable service, few persons ever give the rod any credit. Every 
leaf of the forest and every blade of grass is a pointed conductor 
acting in the same way. 

(&.) There is some question as to the space protected by a rod, but 
the following is a good rule : The protected space is a cone having 
its apex at the tip of the rod and having a base the radius of which 
is equal to the height of the cone. 

370. The Aurora Borealis. — The aurora borealis 
or ^'northern light '^ is frequently seen in northern re- 
gions; beyond the Arctic circle it is of almost nightly 
occurrence. Sometimes its streamers of light radiate like 
the ribs of a fan or form an arch across the northern sky, 
as shown in Fig. 156. But, as seen in this country, it 
more often appears as a few streamers of a pale tint. 
Similar lights are seen in south polar regions and are 
called aurora australis. 

The atmosphere, in its upper strata, is highly rarefied 
and conducts electricity as do the rarefied gases in Geissler 



FRiCtiONAL ELECTRICITY, 



23d 



tubes (Exp. 70). There is little doubt that the aurora is 
due to electric discharges in tliis rarefied air. The appear- 




FiG. 156. 

ance of an aurora is generally accompanied by a ^' mag- 
netic storm" or irregular disturbance that affects all of 
the compass needles over a considerable part of the earth. 

371. Apparatvis and Experiments. — It is 

neither necessary nor very desirable that all of the follow- 
ing experiments be performed. Several of them involve 
the same principle ; but one teacher may have one piece of 
apparatus and another, another piece. Additional experi- 
ments may be found in " The First Principles of Natural 
Philosophy/' pp. 174-176. 

Experiment 38. — Place a tin plate containing a handful of small 
bits of tissue paper upon the prime conductor of an electric machine. 
Work the machine and thus produce an imitation snow storm. 



240 



FRICTION AL ELECTRICITY, 



Experiment 39. — The " metallic plates and dancing images " are 
represented in Fig. 157. The images are made of pith. The upper 
plate is in communication with the prime conductor ; 
the lower one, with the earth. When the machine 
is worked, the images dance in a very ludicrous 
manner. Explain. Pith balls may be substituted 
for the images, the resulting phenomena being known 
as ''Yolta's hail." The experiment may be simplified 
by electrifying the inner surface of a glass tumbler 
by rubbing it upon the knob of the prime conductor 
and placing the tumbler over some pith balls on the 
table. 

Experiment 40. — Place a dozen pith balls or some 
Fig. 157. bits of tissue paper on a table between two books 
about 2 inches (5 cm,) thick. Place a pane of glass 
upon the books as shown in Fig. 158. 
Rub the upper surface of the glass 
with the silk pad mentioned in § 302 
(or a silk handkerchief) and notice 
the lively dance of the pith balls. Fig. 158. 





Experiment 41. — In the "electric chime," represented in Fig. 
159, the outer bells are to be put into communication with the prime 
conductor; the central bell is in communication with the earth. 





Fig. 159. 



Fig. 160. 



The clappers are suspended by silk threads. Work the machine 
slowly ; the bells will begin to ring. Explain, 



FmCTIOKAL ELECTRICITY. 



241 




Experiment 42. — In the "Leyden jar and bells/' shown in Fig. 
160, the left-hand bell is in communication with the outer coat of the 
jar ; the clapper is suspended by a silk thread. When the jar is 
charged and placed in position as represented, the bells begin to 
ring and continue to do so for a considerable time. Explain. 

Experiment 43. — In the "electric swing," shown in Fig. 16L, 
the boy is suspended by silk cords. One of the 
insulated knobs is in communication with the 
earth ; the other with the prime conductor. 
When the machine is worked, the boy swings to 
and fro. Explain, 

Experiment 44.— If a pupil hold a Leyden 
jar by the outer coat and, by a wire, connect the 
knob of the jar with the prime conductor, his pjQ j^j^ 

knuckle will attract the balanced lath (Exp. 5) 
when the machine is worked. Explain. 

Experiment 45. — Fasten a small paper kite by a linen thread to 
the prime conductor. When the machine is worked, the kite will 
float around the knob. Explain. 

Experiment 46. — Fasten one end of a long, small, copper wire to 
the prime conductor. Near the other end of the wire, tie a silk cord 
and hang it from the ceiling or other support so that the end of the 
vertical part of the wire shall be at a convenient height. To this 
end of the wire attach a tassel about four or five inches long made 
of many strips of light tissue paper. Work the machine and the 
leaves will diverge. Explain. Extend toward it your clenched fist ; 
the leaves seek the fist. Explain. Instead of your fist, hold a needle 
toward the tassel ; it will be blown away. Explain, Hold the 
needle upright under the tassel. The strips will collapse. Explain. 

Experiment 47. — Stand upon the insulating stool and place your 
left hand upon the prime conductor of the electric machine. Hold in 
your right hand a sewing- needle with the tip of the forefinger cover- 
ing the end of the needle. Bring the right hand cautiously near the 
gold-leaf electroscope. Notice the divergence of the leaves. Now 
uncover the point of the needle and bring it near the electroscope. 
Notice the marked and immediate increase in the divergence of the 
leaves. Explain. 

Experiment 48. — Place an "electric whirl" (which consists of 
a set of horizontal wire arms radiating from a pivot-supported centre, 



242 



FRICTIONAL ELECTRICITY. 





the pointed ends being all bent in the same direction) upon the prime 
conductor. Work the machine and the arms will revolve. (Fig. 
162.) Explain. 

Experiment 49.— The ''electric or- 
rery," represented in Fig. 163, is a pret- 
ty modification of the " electric whirl." 
The short, balanced bar is provided with 
a pointed conductor to produce rotary 
motion upon its supporting pivot, which 
is one end of the long balanced bar. 
This longer bar 
is also provided 
with a pointed 
conductor and 
supported i n 
turn upon a 
pivot, which 
Fig. 162. niay be attached Fig. 163. 

to the prime 
conductor. When the machine is worked, the long bar revolves 
upon its fixed pivot ; the short bar revolves upon its moving pivot. 

Experiment 50. — Half fill a wide, glass vessel with water. Within 
this, place a glass beaker and fill it to the same level with water. 
By a wire, connect the water in the outer vessel with the earth ; in 
similar manner, connect the water in the beaker with the electric 
machine. Give the handle of the machine a single turn. Dipping 
one finger into the outer water and another into the inner water, a 
shock is felt. Explain. 

Experiment 51. — Let a pupil stand upon an insulating stool (a 
board supported by four warm tumblers will answer) and place his 
left hand upon the prime conductor. Let him, with his right hand, 
clasp the left hand of another pupil not insulated, their hands being 
prevented from actual contact by an intervening sheet of india-rub- 
ber cloth. After the machine has been worked a moment, let the 
insulated pupil remove his left hand from the prime conductor and 
clasp the free hand of his companion. At this moment of clasping 
hands, a shock will be felt Explain. 

Experiment 52. — Cover one knob of the discharger with gun cot- 
ton sprinkled with powdered rosin. When the Ley den jar is dis- 
charged with this discharger, the cotton and rosin are ignited. 



PmCTTONAL ELECTRICITY, 



243 




Bring the covered knob of tlie discharger into contact with tlie knob 
of the jar with a quick motion. 

Experiment 53. — The *' electric bomb," represented in Fig. 164, 
may be made of ivory, lieavy glass, or thorough- 
ly seasoned wood. The ends of the two metal 
wires are rounded and placed a short distance 
apart. The bomb may be filled with gun- 
powder. One wire is connected by a chain 
with the outer coat of a charged Leyden jar. 
The other wire is to be connected with the 
inner coat by a wet string and the discharger. 
The spark between the ends of the two wires 
ignites the powder. Then try the experiment 
with air instead of powder. 

Experiment 54. — Fig, 165 illustrates a method of igniting an 

inflammable liquid, like 
ether or alcohol, by the 
electric spark. Through 
the bottom of a small 
glass vessel, a, passes a 
metal rod, having a knob 
at its upper extremity. 
The lower end of this 
rod may be brought into 
electrical connection with 
the outer coat of a Ley- 
den jar. Enough ether 
or alcohol is poured into 
a just to cover the knob. 
When the jar is dis- 
charged in the way 
shown in the figure, the 
spark ignites the liquid. 
If alcohol is used, it may 
have to be warmed to 
render the experiment 
successful. 

Experiment 55. — Let 

Fig. 165. a pupil, standing on an 

insulating stool, become 

charged by holding one Land on the prime conductor when the 





244 FRICTIONAL ELBCTUICITY. 

macliine is in operation. If he then bring his knuckle to a metal 
burner from which a jet of gas is issuing, a spark will pass be- 
tween the knuckle and the burner, igniting the gas. An Argand 
or Bunsen burner answers well for this experiment. The experi- 
ment may be modified by using, instead of the knuckle, an icicle 
held in the hand. The gas burner may be replaced by a pupil (not 
insulated) holding a spoonful of ether or of chloroform which readily 
gives off an easily combustible vapor. 

Experiment 56. — The "universal discharger," shown in Fig. 166, 
consists of a glass table and two insulated metal rods. (See § 358 a.) 

Balls, points and pincers are 
provided for use at the adja- 
cent ends of the rods which are 
supported upon sliding and 
hinged joints, so that they may 
be easily placed in any desir- 
able position. Cover the ad- 
jacent ends of the two rods 
Fig. 1 66. with metal balls and place them 

upon the glass table, a small 
distance apart. Connect the balls by a very fine wire. One of the 
rods is to be connected by a wire or chain with the outer coats of a 
powerful battery ; the other rod is to be connected, by the discharger, 
(Fig. 150) with the inner coats of the battery. The current thus 
passed along the fine wire may heat it to incandescence, melt or 
even vaporize it. 

Experiment 57. — Prick a profile portrait of Franklin or some other 
design in a sheet of thin card board. Paste two pieces of tin-foil to 
the ends of the card and join them with a piece of gold leaf placed 
over the pricked design. Place a piece of white paper or silk on the 
other side of the card and have the whole tightly screwed up be- 
tween two boards, leaving the edges of the tin-foil strips accessible. 
Discharge a Leyden battery through the gold leaf, thus volatizing 
it, sending the disintegrated particles through the holes in the card 
board and obtaining an impression of the portrait. 

Experiment 58. — Fig. 167 represents ''Volta's pistol," which 
consists of a metal vessel through one side of whi<3h passes an in- 
sulated metal rod with knobs at both ends. The knob at the inner 
end of this rod is near the opposite wall, so that a spark may easily 
be made to pass between the knob and the body of the pistol. The 
pistol being filled with a mixture of illuminating gas and common 



FRlCTTONAL ELECTRICITY. 



245 



air in equal volumes or with oxygen and hydrogen in the proportion 
of one volume of the former to two of the latter and, the mouth being 
closed by a cork, the passage of the spark brings 
about a chemical union of the mixed gases, a 
violent explosion ensues and the cork is thrown 
some distance. The spark may be produced by 
holding the pistol in the hand and bringing the 
outer knob near the prime conductor ; or the 
pistol may be suspended from the prime con- 
ductor by a wire or chain and the pistol then 
touched with the hand. The pistol may also be 
fired by means of the electrophorus. 

Experiment 59. — On the glass table of the 
universal discharger (Fig. 166), place a piece of ■^^^- ^^7- 

wood and bring the knobs of the sliding rods 
against its ends so that the line joining the knobs shall be in 
the direction of the fibers of the wood. Through the apparatus thus 
arranged, discharge a powerful Leyden jar battery. The piece of 
wood will be torn in pieces. 

Experiment 60. — Support a pane of glass upon a glass cylinder, 
in the axis of which is a pointed conductor that just touches the 





Fig. i68. 



pane. On the upper side of the pane directly over this pointed con- 
ductor, place a drop of oil to prevent the spark from gliding over 
the surface of the glass instead of passing through. From an in- 



246 



FitiCTIONAL DLECTitiCITT. 



sulated support, lower a second pointed conductor until it touches 
the pane at the oil. Through these two pointed conductors (Fi|y. 
168), discharge a Leyden jar or 'battery. Unless the glass is mry thin, 
a single jar will not be sufficient. If the experiment fails the first 
time, do not use the same piece of glass for the second trial. A plate 
of glass, 6 cm. thick, has been pierced by means of a powerful in- 
duction coil. 

Experiment 61. — With corks, plug the ends of a glass tube filled 
with water. Through the corks, introduce copper wires until the 
ends in the water are within a quarter of an inch of each other. 
Through these wires, discharge a Leyden jar. The mechanical shock 
due to the repulsion of the electrified water molecules will often 
break the tube. 

Experiment 62.— Charge a Leyden jar. In discharging it, hold 
a stiff card between the knob of the jar and the knob of the dis- 




FiG. 169. 

charger, A hole will be pierced through the card. By the side of 
this hole in the card, make another with a pin. Any one can tell 



PRICTIONAL ELECTRICITY. 



247 



by examination of the pin hole from which side of the card it was 
pierced ; it is burred on only one side. Not so with the perforation 
made by this discharge ; it is burred on both ddes. 

Experiment 63. — One of the inevitable experiments with an elec- 
tric machine consists in " drawing sparks " from the conductor by 
the hand (Fig. lG9j. When the potential of the separated electrici- 
ties becomes sufficient to overcome the resistance of the intervening 
air, they recombine with a sharp, explosive sound and brilliant flash 
of light. (§ 362.) 

Experiment 64. — Divide a circle 
into black and white sectors, as shown 
in Fig. 170, and attach it to a whirl- 
ing table (§ 74). Revolve it so rapidly 
that the colors blend and the disc ap- 
pears a uniform gray. Darken the 
room and illuminate the rapidly re- 
volving disc by the electric spark 
from a Ley den jar. The disc will 
appear at rest and each sector will 
appear separate from its neighbors. 
This shows that the duration of the 
electric spark is less than the persist- 
ence of vision. 




Fig. 170. 



Experiment 65. — In a dark room, place a piece 
of loaf sugar in contact with the outside coat of a 
charged Leyden jar. Place one knob of the dis- 
charger upon the sugar and bring the other near 
the knob of the jar. When the jar is discharged 
thus through the sugar, the sugar will glow for 
some time. 

Experiment 66. — The ''luminous jar," repre- 
sented in Fig. 171, is a modified Leyden jar. The 
outer coat consists chiefly of a layer of varnish 
sprinkled over with metallic powder. A strip of 
tin-foil at the bottom affords means of communica- 
tion with the earth. A similar band at the upper 
edge of the outer coat is provided with an arm, as 
shown in the figure. The rod of the jar is curved 

so as to bring the knob near the projecting arm of the outer coat. 

The jar is suspended by the curved rod from the prime conductor 




248 



FRICTIONAL ELECTRICITY. 




Fig. 172. 




Fig. 173. 



and its lower strip of tin-foil connected 
with the earth. When the machine is 
worked, sparks pass between the knob 
and the projecting arm. In a dark room, 
the metallic powder coat will be beauti- 
fully illuminated at the passage of each 
such spark. 

Experiment 67.— The 'Muminous 
pane" is represented in Fig. 172. A 
continuous tin-foil strip is pasted back 
and forth upon the surface of a plate of 
glass. The upper end of this strip is con- 
nected with the prime conductor ; the 
lower end with the earth. A series of 
breaks in this continuous conductor may- 
be made by cutting it across with a sharp 
pen-knife. When the machine is worked, 
a small spark will appear at each break 
thus made. These breaks may be ar- 
ranged so as to represent a flower, star, 
arch, word or other de- 
sign. The sparks are 
really successive, but 
they seem to be simul- 
taneous. 



Experiment 68. — 

The *' luminous globe " 
is represented in Fig. 
173 and the " luminous 
tube "in Fig. 174. The 
first of these consists 
of a hollow glass globe, 
on the inner surface of 
which small discs of 
tin-foil are placed very 
near each other. The 
first disc is in connec- 
tion with the prime 
conductor, and the last 
one with the ground. 
When the machine is 




Fig. 174. 



FRICTIONAL ELECTRICITY, 



349 



worked, bright sparks appear at each break between the discs. The 
construction and action of the luminous tube are similar. All of 
these luminous effects are best exhibited in the dark. 

Experiment 69.— If two barometer 
tubes, united at the top, be filled with 
mercury and inverted over two cups of 
mercury, as shown in Fig. 175, a Torri- 
cellian vacuum will be formed at the 
bend. When the mercury of one cup is 
connected with the prime conductor and 
that of the other with the earth, the up- 
per part of the tube (containing only mer- 
curic and other vapors) is filled with light. 
The luminosity may be increased by 
raising the temperature and thus in- 
creasing the density of the aeriform con- 
ductor. (A true vacuum will not con- 
duct electricity.) The apparatus may 
be put into the circuit of an induction 
coil instead of connecting it with the 
prime conductor and the earth. 

Experiment 70. — "Geissler's Tubes" 
are sealed glass tubes containing a 
highly rarefied vapor or gas. Platinum 
wires are sealed into the glass at each 
end, to conduct the electric current to 
the interior of the tube. The brilliancy 
and beauty of the light, the great variety 

of effects, color and fluorescence, are indescribable. They are made 
in great variety of form and size and filled with rarefied vapors and 
gases f)f many kinds. A few of the forms are represented in Fig. 




— tf-^'^^ — 

Fig. 176. 

176. They may be used in the dark with an electric machine or an 
induction coil (§ 459). 



250 



FRICTIOKAL ELEOTUtClTY. 



Experiment 7l.~In ''Crookes's Tubes/' devised in many forms 
by Prof. Crookes for his investigations of the phenomena of '' radiant 
matter" (§ 59 6), the tension of tlie contained gas is reduced to about 
one millionth of an atmosphere, far below that of Geissler's tubes. 
Under the inflaence of the electric discharge, matter seems to bo 
radiated from the negative pole in straight lines' and in directions 
perpendicular to the radiating surface. 




Fig. 177. 

(«.) One of these tubes, used to show that *■ radiant matter" may 
exert mechanical action, is shown in Fig. 177. It consists of a highly 
exhausted glass tube containing a glass railway. The axle of a 
small wheel revolves on the rails, the spokes of the wheels carrying 




Fig. 178. 



mica paddles. Pole pieces are fused in through the glass, as repre- 
sented. Whichever pole is made negative, /' radiant matter " darts 



FRICTIONAL ELECTRICITY. 251 

from it along the tube, strikes the upper paddles, causing the wheel 
to roll along the railway. By reversing the poles, the motion of the 
wheel may be stopped and reversed. 

(6.) To show that "radiant matter'' may be deflected from a 
straight line, he devised the tube shown in Fig. 178. The negative 
pole, a 6, is in the form of a shallow cup. A mica screen, c, shields 
the mica paddle-wheel, e f. By holding one pole of the magnet, g, 
over the tube, the matter radiated from a b is deflected upward and 
the wheel caused to revolve like an overshot water-wheel. By hold- 
ing the other pole of the magnet over the tube, the molecular stream 
is deflected downward and the wheel caused to revolve as an under- 
shot water-wheel. (See Appendix C.) 

373. Relation of Electricity to Energy. — 

The work necessarily performed in operating an electric 
machine is not all expended in overcoming inertia and 
friction. Much of it is employed in producing electric 
separation. It matters not whether this separation be 
the separation of two fluids or of something else. WTiat- 
ever he the nature of the realities separated, me- 
chanieal, hinetie energy is employed in the separa- 
tion and converted into the potential variety (§ 159). 
An electrified pith ball or a charged Leyden jar is simply 
an electrostatical reservoir of potential energy. In the 
discharging of such a body, the passage of the current is 
accompanied by a loss of potential energy. What becomes 
of this energy ? This leads us to look for effects due to 
it, to work done by it. Many illustrations of work thus 
done have been furnished in the experiments just de- 
scribed. In eyery case of electric attraction or repulsion, 
we have an evident reconversion of this potential energy 
into mechanical kinetic energy. We shall soon see that 
the sound, heat and light accompanying electric dis- 
charges are forms of energy due to the conversion of the 
potential energy of electric separation. We shall see other 



252 FRICTIONAL ELECTRICITY. 

effects, more or less powerful, when we come to study- 
voltaic and other forms of current electricity. 

Exercises. 

1. (a.) If a gold-leaf electroscope be placed within a tin pail which 
is insulated and electrified, what will be the action of the electro- 
scope? (6.) Explain. 

2. {a.) Why may one obtain a stronger spark from a Leyden jar 
than from the machine by which it is charged? (6.) A Leyden jar 
standing upo^;i a glass plate cannot be strongly charged. Why ? 

3. (a.) A globe that is polished will remain electrified longer than 
one that is not polished. Why? (&.) Can you devise an appendage 
to the outer coat of a Leyden jar, so that it may be charged when 
standing upon a plate of glass ? 

4. (a.) Describe the plate electric machine. (5.) Explain its ac- 
tion, (c.) Explain the action of the electrophorus. 

5. {a.) A minute after the discharge of a Leyden jar, a second jxnd 
feebler spark may generally be obtained. Explain, (p.) State two 
uses of lightning-rods. 

6. {a.) Having a metal globe positively electrified, how could you 
with it negatively electrify a dozen globes of equal size without 
afiecting the charge of the first ? (&.) How could you charge posi- 
tively one of the dozen without affecting the charge of the first ? 

7. Can you devise a plan by which a series of Leyden jars, placed 
upon a glass plate, may be simultaneously charged, the first posi- 
tively, the second negatively, the third positively, the next nega- 
tively and so on ? 

8. How would you prove that there is no electrification within a 
closed conductor? 

9. At what distance from a small sphere charged with 28 units of 
electricity must you place a second sphere charged with 56 units 
that one may repel the other with a force of 32 dynes ? Ans. 7 cm. 

10. If a number of Leyden jars be separately charged in the or- 
dinary way and then connected in series, so that the outer coating 
of one is connected with the inner coating of the next, will the po- 
tential of the battery be changed and in what way ? 

11. Will the *' striking distance" of a battery of Leyden jars in 
series be less or greater than the striking distance (i.e., the greatest 
distance at which the discharge by spark will take place through 
air) of a battery of the same number of similar cells arranged abreast 
as shown in Fig. 152 ? 



FRICTIONAL ELECTRICITY. 253 

12. In what way may an electric charge be divided into three 
equal parts ? 

13. Suppose two similar conductors to be electrified, one with a 
+ charge of 5 units and the other with a — charge of 3 units. 
They are made to touch each other. When they are separated, 
what will be the charge of each ? 

Ans. One unit of + electricity. 

14. Why are telegraphic signals through a submerged cable re- 
tarded in transmission ? 



254 



FRICTIONAL ELECTRICITY, 



Recapitulation. — To be amplified by the pupil for 
review. 

r KINDS AND NAMES. 

ELECTROSTATIC LAWS. 

ELECTRICAL UNITS AND TESTS. 

ELECTROSCOPES. 

CONDUCTION... 



TENSION, POTENTIAL AND CAPACITY. 

r BY CONTACT. 



O 

H 
U 

>< 

W 

Q 
u 

t) \ PROVISIONAL THEORY. 



CONDUCTORS, NON-ELECTRICS. 
INSULATORS, ELECTRICS. 



ELECTRIFICATION. 



BY INDUCTION. 



POLARIZATION. 
ELECTROPHORUS. 
ELECTRIC MACHINES. 
^ SOURCE OF ENERGY. 



O 

h 
u 

U 



DISTRIBUTION OF j on surface. 



CHARGE. 



CONDENSERS. 



DISCHARGE. 



1 density. 

dielectrics, 
inductive capacity, 
leyden jar. 
leyden battery, 
submarine cables. 

disruptive. 

convective. 

conductive. 

THUNDER STORMS, i lightning. 

\ LIGHTNING- 



ATMOSPHERIC E. 

' AURORA BOREALIS. 

I RELATION TO ENERGY. 



LIGHTNING-RODS 



Section hi. 

9 . ^ • 



VOLTAIC AND THERMO-ELECTRICITY. 

373. Chemical Action. — All chemical changes are 
accompanied by electric separation. The substances acted 
upon may be solid, liquid or aeriform, but the chemical 
action between liquids and metals gives results the most 
satisfactory. Electricity thus developed is called voltaic 
or galvanic electricity. Its energy is derived from the 
potential energy of chemical affinity (§ 7). 

374. Current Electricity. — The principal classes 
of electric currents are as follows: 

(1.) Currents produced hy chemical action, i. e., 
voltaic electricity. 

(2.) Currents produced hy heat, i. e,, thermo- 
electricity. 

(3.) Currents produced by other electric currents 
or hy magnets, i. e., induced electricity, 

(a.) We have seen that, when a body having an electrical charge 
is properly connected with another of lower potential, there is a 
transfer of electricity from the former to the latter. This implies 
that there is an electric current. But this current is only momentary 
and of little importance in comparison with the currents that we are 
about to consider. Current electricity may differ from static elec- 
tricity in quantity, electromotive force, etc., but not in its nature. 

375. The Voltaic Current. — When a strip of 
copper and one of zinc are placed in dilute sulphuric acid 

25 




256 VOLTAIC ELECTRICITY. 

or in a battery solution like the one already used, the two 
strips being connected above the acid by a wire conductor, 

a current of electricity is produced. 

T]%6 apparatus here described 

is called a voltaic or galvanic 

element or cell. 

{a ) For voltaic purposes, the sulphuric 

acid should be diluted by slowly pouring 

the acid into ten or twelve times its bulk 

of soft water. Do not pour the water 

^^^^ into the acid. 

Fig 179. 3^g Whence the Energy 

of Current ? — The energy of the current is due to the 
potential energy of chemical affinity existing between the 
acid and the zinc. As the chemical affinity between coal 
and oxygen develops, in the furnace^ a form of kinetic en- 
ergy that we call heat^ so the potential energy of chemical 
separation between the acid and the zinc develops, in the 
cell, the two varieties of kinetic energy, heat and electric 
current. The coal is consumed in the one case ; the zinc, 
in the other. 

377. Direction of the Current. — For this pro- 
duction of the electric current, it is necessary that the 
liquid have a greater action upon one plate than upon the 
other. The plate that is more vigorously acted upon by 
the liquid constitutes the generating or positive plate ; the 
other, the collecting or negative plate. This relation of 
the plates determines the direction of the current. In 
the liquid, the current is from the positive to the 
negative plate; in the wire, the current is from 
1fhe positive to the negative electrode. In each 



VOLTAIC ELECTRICITY. 257 

case^ the current passes from + to —. The direction 
of the current is indicated by arrows in Fig. 179. 

When the wires from the two plates are in contact^ it 
is said that the circuit is closed ; when the plates are not 
thus in electrjc connection, it is said that the circuit is 
broken, 

378. Electrodes. — It may help the memory to sup- 
pose that, in a voltaic cell, two currents, opposite in kind 
and direction, are simultaneously produced. It will be 
readily understood, by keeping in mind the direction of 
these two currents, that, if the circuit be broken, negative 
electricity will accumulate at the end of the wire attached 
to the positive plate and positive electricity at the end of 
the wire attached to the negative plate. These ends of 
the wires are then called poles or electrodes. The 
negative pole is attached to the positive plate and 
vice versa. The plate or electrode from ivhich the 
current flows is + ; that toward ivhich the current 
flows is — . Strips of platinum are often fastened to the 
ends of the wires ; these platinum strips then constitute 
the electrodes. 

379. Resistance. — Every electric circuit offers a re- 
sistance to the passage of the current. This resistance 
will, of course, depend largely upon the materials used for 
the circuit. (See Appendix K.) 

( 1.) With a conducting wire of a given material^ 
the resistance is proportional to the length. If the 
resistance of a mile of telegraph wire be 13 ohms, the re- 
sistance of 50 miles of such wire will be (13 ohms x 50 =) 
650 ohms. 

( 2. ) With a conducting wire of a given material, 



258 VOLTAIC ELECTRICITY, 

the resistance is inversely proportional to its sec- 
tional area, to the square of its diameter or to its 
weight per linear unit. If one conductor be twice the 
diameter of another made of the same length and material^ 
the sectional area or the weight per foot or yard will be 
(2^ = ) four times as great and the resistance of the first 
will be one-fourth as great as that of the second. If they 
be made of the same material and length, one weighing 
twice as much per foot as the latter, the resistance of the 
former will be half as great as that of the latter. (See 
Appendix I.) 

(3.) The resistance of a conducting wire of given 
length and thichness depends upon the material 
of which it is made, i. e,, upon the specific resist- 
ance of the material. (See Appendix K, [2].) 

(4.) The resistance of a given conductor may vary with 
its temperature. (See Appendix K, [3].) 

{a.) Conductivity and resistance are reciprocals, but it is more 
common to speak of the resistances of conductors than of their con- 
ductivities. 

380. The Practical Unit of Resistance.— 

The practical unit of resistance is called an ohm. 
A megohm is a million ohms, A microhm is one- 
millionth of an ohm. The ohm is the resistance of a 
column of mercury one square millimeter in section and at 
the freezing temperature (0° 0.). The exact length of this 
column is to be determined experimentally by an interna- 
tional commission. A recent determination of the value 
of the ohm (probably the best yet made) gives the mercury 
column a length of 106.3 cm. If the pupil will get, from 
some dealer in electrical supplies^ 40 ft. of No. 24 insulated 



VOLTAIC ELECTRICITY, 259 

copper wire (see Appendix I), he will have a very good 
standard ohm. 

(a.) A galvanized iron (telegraph) wire, 4 millimeters in diameter 
and 100 meters long, or a pure copper wire, 1 millimeter in diameter 
and 48 meters long, has a resistance of about one ohm. An ohm 
equals 10^ absolute electro -magnetic units (§ 452). (For the measure- 
ment of resistances, see Appendix M, [2 and 3].) 

381. Kxainples. — {ci^ If the resistance of 130 yd. of copper 
wire, Jg inch in diameter, be one ohm, w^liat is the resistance of 260 
yd. of copper wire, ^^ inch in diameter? Since the diameter of the 
first wire is twice that of the second, the sectional area of the first will 
be four times that of the second. (Areas of circles are proportional 
to the squares of their diameters.) Therefore, the resistance of the 
same length (130 yds.) of the smaller wire will be four times that of 
the larger wire, or 4 ohms. But the second or smaller wire is twice 
as long. Therefore, its resistance will be twice (ff J) as great, or 8 
ohms. Arts. 8 ohms. 

{b.) What is the resistance of 20 yd. of platinum wire, 0.016 inch 
in diameter, if the resistance of 200 yd. of copper wire, 134 mils in 
diameter, is 0.34 ohm and the relative resistances of platinum and 
copper are as 11.3 : 1? (A mil is the one -thousandth of an inch. 
The term is frequently used in descriptions of wire.) 

0.34 ohm X —— X I -—- 1 X — — = 26.95 ohms. 
200 \ lo / 1 

Arts. 26.95 ohms. 

383. Electromotive Force. — Electromotive force 
(often written E. M, F, or simply E) is the mysterious 
power that causes a transfer of electricity from one point 
to another. It is somewhat analogous to hydrostatic pres- 
sure. Wherever there is difference of potential, there is 
E. M, F, The terms are not synonymous^ although, for 
convenience, E. M, F. is often expressed as difference of 
potential and vice versa. The E. M. F, of a yoltaic 
cell depends upon the nature of the materials used and 
not upon the size of the plates or the distance between 
them. 



260 



VOLTATC ELECTRICITY. 



The unit of electromotive force is called a volt. 
A TVhicrovolt is one-millionth of a volt. 

A volt is a little less than the E. M. F. of a Daniell cell 
(§ 394), which measures 1.079 volts. 

(a.) A volt equals 10^ absolute electromagnetic units (§ 452). 
(For the measurement of E. M. F. see Appendix M, [4].) 

383. Internal Resistance. — We may imagine 
that the two plates of a voltaic cell are connected by a 
liquid prism. The greater the distance between the plates, 
the longer this prism and the greater its resistance. The 
larger the plates, the larger the prism and the less its re- 
sistance. (See Appendix M, [3].) 

When the circuit is closed, hydrogen is set free by the 
decomposition of the liquid and rises from the surface of 
the negative plate. Gases are poor conductors. Hence, 
the hydrogen bubbles that often adhere to the negative 
plate increase the internal resistance of the cell by lessen- 
ing the effective surface of the plate (§ 389). This ten- 
dency of the hydrogen to adhere to the plate is one of 
the practical difficulties to be overcome in working a 
voltaic cell or battery. 

384. Fall of Potential.— The existence of a cur- 
rent is evidence of a 
difference of potential 
at any two consecu- 
tive points of the cir- 
cuit. It may be well 
to compare the flow 
of electricity with the 
flow of water in hori- 
zontal pipes and difference of potential with difference of 




Fig. i8o. 



VOLTAIC ELECTRICITY. 



361 




Fig. i8i. 



hydrostatic pressure. Let Fig. 180 represent a vessel filled 
with water. The tap at O is closed and the water stands at 
the same level in all of the vertical tubes (§ 234) showing 
that there is no difference of pressure and, consequently, no 
liquid flow\ Similarly, 
when there is no differ- 
ence of potential there is 
no electric flow. But 
when the tap at C is 
opened, as represented 
in Fig. 181;, it is noticed 
that the level in the ver- 
tical tubes becomes lower as we pass from A toward C. The 
height of water in each vertical tube indicates the pressure 
at that part of the tube, B. This difference in hydrostatic 
pressure produces a flow of water. In much the same way, 
if the electric potential of a voltaic circuit be measured at 
different points, it will be found to decrease from the + pole 
to the — pole. If the circuit be a wire of uniform size 
and material, the resistance offered by it will be uniform 
and the potential will fall uniformly. If, however, the cir- 
cuit be made to have a varying resistance in different parts, 
the potential will fall most rapidly along the parts of 
greatest resistance. For the whole or any part of the 
circuit, the fall of potential will be proportional to the 
resistance. 



{a.) A number of hydraulic motors may be worked "in series" 
upon a given water pipe, tbe outflow of the first being tlie supply of 
the second. The work done in any motor may be determined from 
the quantity of water flowing through the pipe or motor per ij-econd 
and the difference between the supply pressure and the back pressure 
at the motor. There will be a fall of pressure between the two sides 
of the motor at work. The more work the motor has to do, the more 



263 VOLTAIC ELECTRICITY. 

resistance it will offer to tlie flow of water and the greater the fall of 
pressure. Similarly, a number of telegraphic instruments or electric 
lamps may be placed in series upon an electric circuit. The work 
done in each instrument or lamp will depend upon the current 
strength and the difference of potential between the two terminals 
of the instrument or lamp. There will be a fall of a certain number 
of volts between the two terminals, depending upon the intervening 
resistance. 

385. The Amj^ere. — The strength of current or its 
rate of flow (often called its intensity) will depend upon 
electromotive force and resistance^ increasing with the 
former and decreasing with the latter. The unit of 
current is called an ampere. One-thousandth of 
an ampere is called a milli-ampere. At any given 
instant, the current is the same at every part of the circuit. 

(a.) The telegraphic currents commonly used on main . lines vary 
from 5 to 15 milli-amperes. The currents commonly used in electric 
arc lamps vary from 7 to 20 amperes. 

(6.) The strength of a current may be measured by its heating 
effect (§ 471) or by the products of electrolysis, as in the case of the 
water voltameter (§ 410). But currents are generally measured by in- 
struments like the galvanometer (§ 418), or by their electro-magnetic 
effects. An instrument so used is called an ammeter (abbreviated 

from ampere-meter). An ampere equals 0.1 or 10 of an absolute 
electro-magnetic unit (§ 452). 

386. Ohm's Law, — TJte strength of current 
varies directly as the E, M. F, and inversely as 
the resistance. This resistance is the total resistance of 
the circuity including the internal resistance of the cells 
or dynamo and the resistance of the external circuit. 

.^^— = Amperes, or C = -^.:.E ^ C x R ; R = ^. 

Standards for strength of current have not yet been 
made. 



VOLTAIC ELECTRICITY. 263 

{a.) Ohm's great service (a d., 1827) to electrical science consisted 
largely in the introduction of the accurate ideas, electromotive 
force, current strength and resistance. " Before his time, the 
quantitative circumstances of the electric current had been indicated 
in a very vague way by the use of ^the terms 'intensity' and 
'quantity,' to which no accurately defined meaning was attached." 

(6.) If we have a difference of potential that secures an E. M. F. 
of 18 volts, and if the total resistance of the circuit be 3 ohms, the 
strength of the current will be 6 amperes. 18 -f- 3 = 6. The 
analogy of flowing water will again help us. The rate at which the 
water is delivered will depend upon, not only the head or pressure 
(corresponding to E. M. F.),but also upon the resistance it meets with 
in flowing. If the pipe be small and crooked or if it be choked with 
sand or sawdust, the water will flow in a small stream even though 
the pressure be great. 

Experiment 72. — Make four coils or spools of insulated wire as 
follows : (See Appendix I.) 

No. 1, of 100 feet of No. 16 gauge, copper. 

No. 2, of 100 " " 30 '' 

No. 3, of 50 " ♦' 30 '' 

No. 4, of 50 " "30 *' german silver. 

Place the wire of the first spool and a oralvanometer (§ 418) in the 
circuit of one cell and note the number of degrees of deflection of 
the galvanometer needle. Put the second spool in place of the first. 
The smaller deflection shows that (other things being equal) the 
No. 16 wire transmits more current than the No. 30. Why ? Then 
add the third spool to the circuit. The still smaller deflection shows 
that (other things being equal) a long wire transmits less current 
than a shorter one. Why? Remove the second spool from the 
circuit and note the deflection of the galvanometer. Put the fourth 
spool in place of the third. The diminished deflection shows that 
(other things being equal) a german silver wire transmits less current 
than a copper wire. Why ? With any one of the spools in the cir- 
cuit, compare the galv^anometer deflections produced by a Bunsen 
cell and by a gravity cell and notice that the former gives the 
stronger current. 

Note. — These experiments give very crude results but, such as 
they are, they fairly represent the measurements that prevailed 
until recently. More accurate measurements with numerical repre- 
sentations of the results are now demanded. The rapid advances of 



264 VOLTAIC ELECTRICITY. 

electrical science within the last few decades have been very largely 
due to the adoption of definite units and accurate determinations. 
(See Appendix M.) 

387. The Coulomb. — The unit of quantity is 
called the coulomb. It is the quantity of elec- 
tricity given by a one ampere current in one 
second, A ten ampere current will give thirty coulombs 
in three seconds. 

(a.) The word *' quantity" was formerly used in the sense in 
which the word "intensity" was used in § 385, while the latter 
word was used as if it depended upon El. M. F. alone. But quantity 
of electricity, clearly, depends upon the strength of the current and 
the time that the current flows. A coulomb equals 0.1 or IQ-i of an 
absolute electro-magnetic unit of quantity (§ 452). 

Exercises. 

1. What length of "No. 10 pure copper wire (B. & S.) will have a 
resistance of 1 ohm ? (See Appendix I.) Ans, 961.54 ft. 

2. A given battery has an E. M. F. of 12 volts. The internal 
resistance is 8 ohms. The resistance of the external circuit is 4 
ohms. What is the strength of tlie current ? 

3. The 4 cells of a given battery are connected so that the total 
E. M. F. is 4 volts and the internal resistance is 20 ohms. The 
external circuit has a resistance of 20 ohms. What is the strength 
of the current? Ans. 0.1 ampere. 

4. What length of copper wire A mm. in diameter will have the 
same resistance as 12 yd. of copper wire 1 mm. in diameter? 

Ans. 192 yd. 

5. The 4 cells of a given battery are connected so as to give an 
E. M. F. of 2 volts and to have a total internal resistance of 10 ohms. 
The external circuit is a stout copper wire with a resistance so small 
that it may be ignored. What is the current strength ? 

6. The same battery is used with a telegraphic sounder in the 
circuit. This instrument has a resistance of 5 ohms. What is the 
current strength ? Ans. 133 milli -amperes. 

7. The resistance of 47 ft. of copper wire, 22- mils in diameter 
being 1 ohm, find the resistance of 200 yd. of copper wire 134 mils 
in diameter Ans. 0.34 ohm. 

^^ If you do not know what a mil is, consult the Index. 



VOLTAIC ELECTRICITY. 265 

8. A battery has a current of 2 amperes flowing through a total 
resistance of 9 ohms. What is the E. M. F. ? 

9. The E. M. F. of a battery is 10 volts. The current is 1 ampere. 
The external resistance is 5 ohms. What is the internal resistance 
of the battery ? Ans. 5 ohms. 

10. The potential of a current falls 45 volts between the two 
terminals of an incandescence lamp. The current measures 1.25 
amperes. What is the resistance of the lamp? Ans. 36 ohms. 

^^ If you do not know what an incandescence lamp is, consult 
the Index. 



266 VOLTAIC ELECTRICITY, 

388, Amalgamating the Zinc. — Ordinary com- 
mercial zinc is far from being pure. The chemically 
pure metal is expensive. When impure zinc is used^ small 
closed circuits are formed between the particles of foreign 
matter and the particles of zinc. This local action, which 
takes place even when the circuit of the cell or battery is 
broken, rapidly destroys the zinc plate and contributes 
nothing to the general current. This waste, which would 
not occur if pure zinc were used, is prevented by fre- 
quently amalgamating the zinc. This is done by clean- 
ing the plate in dilute acid and then rubbing it with 
mercury. 

{a.) The method of amalgamating battery zincs practised by the 
author is as f onows : In a glass vessel placed in hot water, dissolve 
15 cw. cm. of mercury in a mixture of 170 cu. cm. of strong nitric 
acid and 625 cu. cm. of hydrochloric (muriatic) acid. When the 
mercury is dissolved, add 830 cu. cm. of hydrochloric acid. When 
the liquid has cooled, immerse the battery zinc in it for a few- 
minutes, remove and rinse thoroughly with water. The liquid may 
be used over and over until the mercury is exhausted. The quan- 
tity here mentioned will suffice for 200 ordinary zincs or more. 
Keep the liquid, when not in use, in a glass-stoppered bottle. 

389. Polarization. — It was stated in § 383 that the 
accumulation of hydrogen bubbles at the negative plate 
increases the internal resistance of the cell. But the 
hydrogen affects the current in another way. It acts like 
a positive plate (being almost as oxidizable as the zinc) 
and sets up an opposing electromotive force that tends 
to set a current in the opposite direction. A cell or hat- 
tery in this condition is said to he polarized. Some- 
times, as a result of polarization, the strength of the cur- 
rent falls off very greatly within a few minutes after clos- 
ing the circuit. (See § 414.) 



VOLTAIC ELECTRICITY, 



267 



390. Varieties of Voltaic Cells.— All voltaic 
cells belong to one of two classes : 

(1.) Tlxose using only one liquid. 

(2.) Those using two liquids. 

All of the earlier batteries were composed of one-liquid 

cells. 

Note. — When dilute sulphuric acid is mentioned in connection with 
cells and batteries, it may be understood that one volume of acid to 
ten or twelve volumes of water is meant. 



391. Smee's Cell.— A Smee's cell 
is represented by Fig. 182. It consists of 
a platinized silver plate placed between 
two zinc plates hung in dilute sulphuric 
acid. The hydrogen bubbles accumulate 
at the points of the rough platinum sur- 
face and are more quickly carried up to 
the surface of the liquid and thus gotten 
rid of. The cell has an available electro- 
motive force of about 0.47 volt. 




Fig. 182. 



392. Potassium Di-chromate Cell. — The po- 
tassium di-chromate cell has a zinc plate hung between 
two carbon plates. A solution of potassium di-chromate 
(bi-chromate of potash) in dilute sulphuric acid is the 
liquid used. The hydrogen is given an opportunity for 
chemical union as fast as it is liberated. The E. M. F. of 
this cell is great to start with (from 1.8 to 2.3 volts) ^ but 
it falls very quickly when the external resistance is small. 
It quickly recovers and may be used with advantage 
w^here powerful currents of short duration are often 
wanted. It is the only single liquid cell that is free from 
polarization. 



268 



VOLTAIC ELECTRICITY, 




(a.) The bottle form of this cell, represented in Fig. 183, is the 
most convenient for the laboratory or lecture table. By means of the 
sliding rod, the zinc plate may be raised out 
of the solution when not in use. Thus ad- 
justed, the cell may remain for months with- 
out any action, if desired, and be ready at a 
moment's notice. 

(5.) One of the best proportions for the solu- 
tion is as follows : One gallon of water, one 
pound of potassium di-chromate and from a 
half pint to a pint of sulphuric acid, according 
to the energy of action desired. A small 
quantity of nitric acid added to the solution 
increases the constancy of the battery by oxi- 
dizing the nascent hydrogen and thus forming 
water. 

(c.) The following recipe is good : Pour 167 
cu. cm. of sulphuric acid into 500 cu. cm. of 
water and let the mixture cool. Dissolve 115 
g. of potassium di-chromate in 335 cu. cm. of 
boiling water and pour, while hot, into the dilute acid. When cool, 
it is ready for use. 

393. The Leclaiiche Cell.— This cell, shown in 
Pig. 184, contains a zinc plate or rod and a porons, earthen- 
ware cup containing the carbon plate. 
The space between the carbon plate 
and the cup is filled with fragments 
of carbon and powdered peroxide of 
manganese. This cup replaces the 
second metal plate. The liquid used 
is a solution of ammonium chloride 
(sal-ammoniac) in water. This cell 
is tolerably constant if it be not used 
to produce very strong currents, but 
its great merit is that it is very permanent. It will keep 
in good condition for months with very little attention, 
furnishing a current for a short time whenever wanted. 




Fig. 184. 



VOLTAIC ELECTRICITY. 



^zm 



It is much used for working telephones, electric bells 
(Fig. 232) and clocks, railway signals, etc. The man- 
ganese oxide prevents polarization by destroying the 
hydrogen bubbles. If the cell be used continuously for 
some time, its power weakens owing to the accumulation 
of hydrogen, but if left to itself it gradually recovers 
as the hydrogen is oxidized. Sometimes the manga- 
nese oxide is applied to the face of the carbon and the 
porous cup dispensed with. This cell has an E. M. R of 
about 1.5 volts. It should he left on open circuit when not 
in use. 




F- 



394. DanielPs Cell. — This cell consists of a copper 
plate immersed in a saturated solution of copper sulphate 
(blue vitriol) and a zinc plate immersed in dilute sul- 
phuric acid or a solution of zinc sulphate (white vitriol). 
The two hquids are separated ; usually 
one liquid is contained in a porous 
cup placed in the other liquid. Cry- 
stals of copper sulphate are placed in 
the solution of copper sulphate to 
keep the latter saturated. Such a 
cell will furnish a nearly constant 
current, with an E. M. F. of 1.079 
volts and keep in order for a long 
time. It should he hept on closed cir- 
cuit when not in tise. The hydrogen passes through the 
porous cell and acts upon the solution of copper sulphate. 
Copper, instead of hydrogen, is deposited upon the copper 
plate. Polarization is thus avoided. If an incrustation 
forms near the zinc plate, remove some of the solution of 
zinc sulphate and dilute what remains with water. 



Fig. 185. 



270 



VOLTAIC lSLt]CTItICITY, 



{a. ) In Fig. 186, the copper plate is represented as a cleft cylinder 
within the porous cap, the crystals being piled up around it. It is 
common to interchange the plates, the zinc being in dilute sulphuric 
acid within the porous cup, and the copper plate in the saturated 
acid outside the porous cup. Sometimes the outer vessel itself is 
made of copper instead of glass and serves as the copper plate as is 
shown in Fig. 185. 



.1.^' 





Fig. i86. 



Fig. 187. 



395 The Gravity Cell. — This is a modification 
of the Daniell's cell, no porous cup being used. The cop- 
per plate is placed at the bottom of the cell and the zinc 
plate near the top. Crystals of copper sulphate are piled 
upon the copper plate and covered with a saturated solu- 
tion of copper sulphate. Water or, preferably, a weak 
solution of zinc sulphate rests upon the blue solution be- 
low and covers the zinc plate. The two solutions are of 
different specific gravities and remain clearly separated if 
the cell be kept on closed circuit when not in use. (Fig. 
187.) This cell is very largely used in working telegraph 
lines. It is sometimes called the Callaud cell. 



396. Grove's Cell. — The outer vessel of a Grove's 
cell contains dilute sulphuric acid. In this is placed a 



VOLTAIC ELB)CTRiCTTY. 



271 



hollow cylinder of zinc. Within the zinc cylinder is 
placed a porous cup containing strong nitric acid. The 
negative plate is a strip of platinum placed in the nitric 
acid. The hydrogen passes through the porous cup and 
reduces the nitric acid to nitrogen peroxide, which escapes 
as brownish-red fumes. These nitrogen fumes are dis- 
agreeable and injurious; it is well;, therefore, to place the 
battery in a ventilating chamber or outside the experiment- 
ing room. The E. M. F. of the Grove cell, under favor- 
able conditions, is nearly two volts, while its internal re- 
sistance is small, being about one-fifth that of a Daniell's 
cell. It is much used for working induction coils (consult 
the Index), for generating the electric light, etc. It is, 
however, troublesome to fit up and should have its liquids 
renewed every day that it is used. Fig. 189 represents a 
Grove's battery with cells joined in sei^ies, 

397. Bllllsen^s Cell.- 

Bunsen's cell (Fig. 188) dif- 
fers from Grove's in the use 
of carbon instead of expensive 
platinum for the negative 
plate, thus reducing the cost. 
The plates are made larger 
than for Grove's battery. Its 
E. M. F. is about the same as 
that of the Grove cell but its 
internal resistance is greater. 
Fig. 190 represents a battery of Bunsen's cells joined in 
multiple arc, 

Note, — There are scores of different kinds of cells in the market 
competing for favor. Those here described are among the ones 
most commonly used. 




Fig. i? 



%n 



VOLTAIC electricity: 



398. A Voltaic Battery. — A number of similar 
voltaic elements connected in such a manner that 
the current has the same direction in all, constitutes 
a voltaic hattery. The usual method is to connect the 
positive plate of one element with the negative plate of 
the next, as shown in Fig. 189. When thus connected, 
they are said to be coupled "tandem'^ or "in series." 
Sometimes all of the positive plates are connected by a 
wire and all of the negative plates by another wire. The 
cells are then said to be joined " parallel/' " abreast '^ or 
"in multiple arc.'' (See Fig. 190.) 

{a.) When two or more cells are joined together, the points of 
contact should be as large as is convenient and kept perfectly clean. 
The connecting wire should be of good size and, for the sake of 
pliability, a part of it may well be given a spiral form by winding it 
upon a pencil or other small rod. 

399. Batteries of High Internal Resist- 
ance, — Each kind of galvanic cell has an internal resist- 




FiG. 189. 

ance, as explained in § 383. A battery of cells joined in 
series is called a "battery of high internal resistance." 
(Fig. 189). This method of joining the cells increases 



VOLTAIC ELECTRICITY, 



278 



the length of the liquid conductor through which the 
current passes. 

(6^.) In a battery of cells joined in series, the E. M. F. and the 
internal resistance are those of a single cell multiplied by the num- 
ber of cells. For a circuit of great external resistance, a battery of 
higli internal resistance is needed. 

400. Batteries of Low Internal Resist- 
ance. — A battery of cells joined parallel is called a 
^'battery of low internal resistance." (Fig. 190.) This 
method of joining the cells does not increase the length 
of the liquid conductor traversed by the current but is 
equivalent to increasing its diameter or sectional area. 

{a) In a battery of cells joined parallel, the E. M. F. is that of a 
single cell, but the internal resistance is that of a single cell divided 
by the number of cells. For a circuit of small external resistance, 
large cells, or several cells joined parallel, are preferable. 




Fig. I go. 

(5.) A battery of high internal resistance was formerly called an 
intensity battery, while a battery of low internal resistance was 
called a quantity battery. 

401. Requisites of a Good Battery. — The 

following conditions should be met by a battery: 

( 1.) Its electromotive force should be high and constant. 
(3.) Its internal resistance should be small. 



274 VOLTAIC ELECTRICITY. 

(3.) It should give a constant current and, therefore^ 
must be free from polarization ; it should not be 
liable to rapid exhaustion, requiring frequent re- 
newal of the acid. 
(4.) It should be perfectly quiescent when the circuit 

is open. 
(5.) It should be cheap and of durable materials. 
(6.) It should be easily manageable and, if possible, 

should not emit corrosive fumes. 
As no single battery fulfills all these conditions, some 
batteries are better for one purpose and some for another. 
Thus, for telegraphing through a long line of wire a con- 
siderable internal resistance in the battery is no great 
disadvantage; while, for producing an electric light, much 
internal resistance is absolutely fatal. 

403. The Best Arrangement of Cells. — The 

best method of coupling cells in any given case depends 
on the work to be done by the battery. The maximum 
effect is attained when the resistance of the ex- 
ternal circuit is made equal to the internal resist- 
ance of the hattery. 

(a.) For example, suppose that in a given battery of eiglit ceUs : 

(1.) Each cell has an E. M. F. of two volts. 

(2.) Each cell has the very high internal resistance of eight ohms. 

(3.) The battery is to work through a wire that has a resistance 
of sixteen ohms. 

(&.) First, couple the cells parallel. The E. M. F. of the battery 
is that of a single cell, 2 volts. The internal resistance is 8 ohms 
-f- 8 = 1 ohm. Adding the external resistance, we have a total 
resistance of 17 ohms. (See § 386.) 

This arrangement gives a current of 0.1176+ amperes. 



VOLTAIC ELECTRICITY. 275 

(c.) Next, couple the cells in series. The E. M. F. of the battery 
is 8 times 2 volts, or 16 volts. The internal resistance is 8 times 8 
ohms or 64 ohms. Adding the external resistance, we have a total 
resistance of 80 ohms. 

_E _ 16 
• ^=i?^64n6^^-^- 
This arrangement gives a current of 0.2 amperes. 

{d.) Finally, join the cells in two rows (each row being a series 
of four cells) and join the rows parallel. The E. M. F. of the battery 
will be 4 times 2 volts or 8 volts. The internal resistance will be 
4 times 8 ohms or 32 ohms for each row, but only half that, or 16 
ohms, for the whole battery. Adding the external resistance, we 
have a total resistance of 32 ohms. 
E 8 

^= ^ = 1-6^16 = ^-^^- 

This arrangement, in which the internal and the external resistances 
are equal, gives a current of 0.25 amperes, the greatest possible 
under the given conditions. 

{e.) A similar application of Ohm's law shows that when the 
external resistance is large^ there is little gain from pining cells 
parallel, and that when the external resistance is very small , there is 
little gain in joining cells in series. 

Exercises. 

1. Given ten cells, each with an electromotive force of 1 volt and 
an internal resistance of 5 ohms. What is the current (in amperes) 
of a single cell, the external resistance being 0.001 ohm ? 

Ans. 0.19996+ amperes. 

2. The ten cells above mentioned are joined abreast. The exter- 
nal resistance is 0.001 ohm. What is the current of the battery ? 

Ans. 1.996+ amperes. 

3. The ten cells above mentioned are joined tandem, the external 
resistance remaining the same. What is the current of the battery ? 

Ans. 0.19999+ amperes. 

4. What is the current given by one of the above mentioned cells 
when the external circuit has a resistance of 1000 ohms? 

Ans. 0.00099502 amperes. 

5. When the ten cells are joined abreast with an external resist 
ance of 1000 ohms, what is the current of the battery ? 

Ans. 0.0009995 amperes. 



276 VOLTAIC ELECTRICITY. 

6. When the ten cells are joined in series with an external resist- 
ance of 1000 ohms, what is the current of the battery ? 

Ans. 0.00992 amperes. 
Note. — Compare the results in Exercises 1, 2 and 3, where we 
have a small external resistance. Then compare the results in Ex- 
ercises 4, 5 and 6, where we have a high external resistance. 

7. Why are cells arranged tandem for use on a long telegraphic 
line ? 

8. What is the resistance of 2 miles of No. 6 electric light wire 
(copper of ordinary commercial quality)? (See Appendix I,) 

Ans. 4.54 ohms. 

9. A Brush dynamo, No. 8, will operate 65 arc lamps on a short 
circuit. Each lamp has a resistance of about 4.52 ohms. If the 
lamps be put on a 10 mile circuit of No. 6 copper wire, how many 
lamps should be " cut out " of the circuit, the dynamo running at 
the same speed and the current strength remaining the same ? 

Ans. 5 lamps. 

10. Show, by a diagram, how a battery of three cells should be 
arranged when the internal resistance is the principal one to be 
overcome. 

11 What is the resistance of a mile of ordinary No. 6 iron tele- 
graph wire? (See Appendix K, [2].) Ans. 12.9 ohms. 

12. Show that the conductivity of water is increased more than 50 
times by adding half its voUime of sulphuric acid. (See Appendix 
K, [2].) 

13. How much is the conductivity of water increased by adding 
^ij- its volume of sulphuric acid? Ans. About 22 times. 



VOLTAIC ELECTRICITY. 277 

403. Long and Short Coil Instruments. — 

A "long coiP' galvanometer, or a ^'long coil'' electro- 
magnet, or an instrument of any kind in whicli the con- 
ductor is a long, thin wire of high resistance, should not 
be employed on circuits the other resistances of which are 
small. Conversely, on circuits of great length, or where 
there is a high resistance, "short coil" instruments are of 
httle service for, though they add little to the resistances, 
their few turns of wire are not enough with the small 
currents that circulate in high-resistance circuits; "long 
coil '' instruments are here appropriate, as they multiply 
the effects of the currents by their many turns. Their 
resistance, though perhaps large, is not a serious addition 
to the existing resistances of the circuit. 

404. Divided Circuits and Shunts. — The case 
of several wires forming a multiple arc often occurs in 
practice. In such cases, the cztrrent flowing iJt each 
branch is inversely proportional to the resistance 
of that branch. Either of two such branches is called 
a shunt. Evidently, the joint resistance of all the branches 
is less than the resistance of any one of them. 




{a.) A current flowing along a conductor divides at A, part goin^ 
through a galvanometer or elect ro-maenet at G and the rest going 
through the branch, B. The currents unite at G. If the conductor, 
AGC, has a resistance of 99 ohms and the conductor, ABC, has a 



278 VOLTAIC ELECTRICITY. 

resistance of 1 ohm, 1 per cent, of the total current will go through 
G and 99 per cent, will go by way of B. 

(b. ) If w^e have two wires, the separate resistances of which are 
respectively 28 ohms and 24 ohms, placed abreast in a circuit, find 
their joint resistance. The joint conductivity will be the sum of the 
separate conductivities and conductivity is the reciprocal of resist- 
ance. Call the joint resistance B. 

-i--i- + -^---?i + ^-^ • 72-^-^-1292 

i^ ~ 28 24 672 672 ~ 672* * 52 ~ 

The joint resistance will be 12.92 ohms. 

(c.) The joint resistance of the two branches of a divided conductor 
is equal to the product of the separate resistances divided by their 
sum. If there are more than two branches, the method employed 
above may be used. 

(d.) It is often necessary to use a sensitive galvanometer or other 
instrument with a current so strong that the current would give in- 
dications too large for accurate measurement or even ruin the instru- 
ment. Under such circumstances, the greater part of the current 
may be shunted around the galvanometer. The resistance of the 
shunt having a known ratio to that of the galvanometer and its 
branch, the total current strength may be computed from the strength 
of the current flowing through the instrument. Shunt circuits may 
be found in almost all arc lamps. 

405. Mechanical Effects of the Electric 
Current. — The piercing of the glass walls of an over- 
charged Leyden jar aflfbrds a good^ though expensive, 
illustration of the mechanical effects of electricity. Trees 
and telegraph poles shattered by lightning are not un- 
familiar. But, by far, more important for our considera- 
tion are the mechanical effects produced by voltaic or 
dynamic electricity and, especially, the numerical rela- 
tion hetiveen the electricity used and the worh done. 
This subject will be considered in Section VI. of this 
chapter. 

Experiment 73. — Through a long, thin platinum wire, send a 
current that will heat it to dull redness. Apply a piece of ice to the 



VOLTAIC ELECTRICITY, 279 

wire and notice that the rest of the wire glows more brightly than it 
did before. Then heat a part of the wire with the flame of a spirit 
lamp and notice that the rest of the wire glows less brightly than 
before. In the first case, the current is strengthened by the in- 
creased conductivity of the cooled part ; in the second case, the cur- 
rent is decreased by the increased resistance of the part heated by 
the lamp. 

Experiment 74.— When two curved metal surfaces rest upon each 
other, a current passing from one to the other encounters considera- 
ble resistance at the small area of contact. The heat consequently 
developed causes the parts in the neighborhood to expand very 
quickly when the contact is made. This often gives rise to rapid 
vibratory movements in the conductors. Gore's railway consists of 
two concentric copper hoops, whose edges are worked very truly 
into a horizontal plane. A light copper ball is placed on the rails 
thus formed. One rail is connected with the 4- pole of a battery 
of two or three Grove cells and the other rail with the — pole. The 
ball is then set rolling around the track. If the ball be true and the 
track well leveled, the energy supplied by the swelling (expansion) 
at the continually changing point of contact is sufficient to keep up 
the motion. The ball will roll round and round, giving a crackling 
sound as it goes. 

Experiment 75. — From the poles of a potassium di-chromate bat- 
tery, lead two stout copper wires and connect their free ends by two 
or three inches of very fine iron or platinum mre. Goil the iron wire 
around a lead pencil and thrust a small quantity of gun-cotton into 
the loop thus formed. Plunge the zinc plate of the battery into the 
liquid and the iron wire will be heated enough to explode the gun- 
cotton ; it may be heated to redness or even to fusion. 

406. Thermal Effects of the Electric Cur- 
rent. — Whenever an electric current flows throngh a 
conductor, part of the electric energy is changed 
into heat energy, TIxe amount of electricity thus 
changed into heat ivill depend upon the amount 
of resistance offered by the conductor. In the last 
experiment^ the stout copper wires were good conductors, 
offered but little resistance and converted but little of the 



280 VOLTAIC ELECTRICITY. 

electrical energy into heat energy. The change of ma- 
terial from copper to iron increased that resistance. This 
increased resistance was again increased by reducing the 
size of the conductor. For this double reason^ the fine 
ivire offered so much resistance that a considerable of the 
current energy was transformed into heat. Resistance 
in an electric circuit always produces heat at the 
expense of the electric current. Thus, electricity is 
often used in firing mines in military operations and in 
blasting. All known metals have been melted in this way, 
while carbon rods have been heated by a battery of 600 
Bunsen's elements until they softened enough for welding. 
By means of a Leyden jar battery and a uniyersal dis- 
charger, remarkable thermal effects may be obtained. 
Houses are sometimes set on fire by lightning. The nu- 
merical relations between electricity and heat are con- 
sidered in Section VI. of this chapter. 



407. Luminous Effects of the Electric 
Current. — The electric spark, the glow seen when elec- 
tricity escapes from a pointed conductor in the dark and 
the various forms of lightning are some of the now 
familiar luminous effects of electricity. Whenever an 
electric circuit is closed or broken, there is a spark at the 
point of contact, due to the heating of a part of the con- 
ductor to incandescence. We have seen Inminous effects 
produced by winding the wire from one plate of a voltaic 
cell round one end of a file and drawing the other electrode 
along the side of the file, thus rapidly closing and break- 
ing the circuit. If the iron wire used in the last experi- 
ment was heated sufficiently, it also gave a luminous effect 



VOLTAIC ELECTRICITY. 



281 



and illustrated the fundamental principle of the incandes- 
cence electric lamp (§ 466). 

{a.) The most important luminous effects of electricity will be 
considered in connection with dynamo-electric machines (§ 465). It 
will be noticed that all of these are secondary thermal effects. 

408. Galvani's Exj)eriiiient. — In 1786, Galvani, 
a physician of Bologna, noticed convulsive kicks in a 




Fig. ig2. 

frog's legs when acted upon by an electric current. A frog 
was killed and the hind limbs cut away and skinned, the 
crural nerves and their attachments to the lumbar vertebrae 
remaining. Two dissimilar metals were held in contact and 
their free ends brought into contact with nerve and muscle 
rospectively, as shown in Fig. 192. Convulsive muscular 
contractions brought the legs into a position similar to 



282 VOLTAIC ELECTRICITY, 

that represented by the dotted lines in the figure. A frog's 
legs thus prepared make a very sensitive galvanoseope. 
It is said that they show even the very feeble induction 
currents of the telephone, though the best galvanometers 
barely detect them. 

409. Physiological Effects of the Electric 
Current. — An electric current may produce muscular 
convulsions in a recently killed animal. Experiments 
with the Leyden jar and the induction coil show that 
similar effects may be produced upon the living animal. 
The "electric shock/' which is physiological in its nature, 
is familiar to most persons. The sensation thus produced 
cannot be described, forgotten or produced by any other 
agency. 

Electricity is largely used as an agent -for the cure of 
disease ; experiments of this kind may do injury and 
would better be left to the educated physician. The dis- 
charge of a large battery may be fatal and a number of 
persons have lost their lives within the last few years by 
coming, accidentally or otherwise, into the circuit of a 
dynamo-electric machine. Interrupted and alternating cur- 
rents are more serious in their physiological effects than 
continuous currents. 

{a.) If the members of a class form a chain by joining hands, the 
first member holding a feebly -charged Leyden jar by its outer coat 
and the last member touching the knob, a simultaneous shock will 
be felt by each person in the chain. A similar experiment may be 
made with a Ruhmkorff coil. A single Leyden jar has been dis- 
charged through a regiment of 1500 men, each soldier receiving a 
shock. Dr. Priestley killed a rat with a battery of seven feet of 
coated surface, and a cat with a battery of forty feet of coated 
surface, 



VOLTAIC ELI^CTRICITY. 



283 



Experiment 76. — Into a bent tube (known to dealers in chemical 
glassware as a U tube), put a solution of any 
neutral salt, e. g., sodium sulphate. Color the 
contents of the tube with the solution from 
purple cabbage. In the arms of the tube, place 
the platinum electrodes of a battery, as shown in 
Fig. 193. Close the circuit and presently the 
liquid at the + electrode will be colored red and 
that at the — electrode, green. If, instead of 
coloring the solution, a strip of blue litmus paper 
be hung near the + electrode it will be reddened, 
while a strip of reddened litmus paper hung near PiG. 193. 

the — electrode will be colored blue. These 
changes of color are chemical tests; the appearance of the green or 
blue denotes the presence of an alkali (caustic soda in this case), 
while the appearance of the red denotes the presence of an acid. 

Experiment 77. — Melt some tin and pour the melted metal slowly 
into water. Dissolve some of this granulated tin in hot hydrochloric 
acid and add a little water. Into this bath of a dilute solution of 
tin chloride, introduce two platinum electrodes from a battery of a 
few cells. A remarkable growth of tin crystals will shoot out from 
the — electrode and spread towards the + , bearing a strong resem- 
blance to vegetable growth. Hence, it is called the "tin tree." 
Repeat the experiment with solutions of lead acetate ("sugar of 
lead ") and of silver nitrate. 



410. Chemical Effects of the Electric Cur- 
rent. — The electric spark may be made to produce chem- 
ical combination or chemical decomposition. Ammonia 
(NH3), or carbon-dioxide (CO 3);, may be decomposed by 
passing a^ series of sparks through it. A mixture of oxygen 
and hydrogen may be caused to enter into chemical union 
by the electric sparky the product of the union being water. 
(See Chemistry, Exp. 53.) Many chemical compounds 
may be decomposed by passing the current through them. 
The compound must be in the liquid condition;, either by 
solution or by fusion. Substances that are thus decom- 
posed are called electrolytes ; the process is called elec- 



284 



VOLTAIC ELECTRICITY, 



trolysis ; the compound is said to be electrolyzed. The 
electrolysis of acidulated water is easily accomplished with 
a current from three or four Grove's or Bunsen's cells. 
The water is decomposed into oxygen and hydrogen. The 
apparatus^ shown in Fig. 194, may be called a water- 
voltamete7\ 




Fig. 194. 



(a.) The apparatus consists of a vessel contaiuing water (to which 
a little acid has been added to increase its conductivity) in which 
are immersed two platinum strips that constitute the two elec- 
trodes of a battery. When the circuit is closed, bubbles of oxygen 
escape from the positive electrode and bubbles of hydrogen from 
the negative. The gases may be collected separately by inverting, 
over the electrodes, tubes filled with water, as shown in the figure. 
The volume of hydrogen thus collected will be about twice as great 
as that of the oxygen. 

(6.) A water- voltameter may be made by cutting off the bottom of 
a wide-mouthed glass bottle {Chemistry, App. 4, h.) and passing two 
insulated wires, varnished and terminating in platinum strips, 
through, a cork that closes the mouth of the inverted bottle. Two 
test tubes will complete the instrument. When a sufficient quantity 
of the gases has been collected, they may be tested ; the hydrogen, 
by bringing a lighted match to the mouth of tbe test tube, where- 
upon the hydrogen will burn ; the oxygen, by thrusting a splinter 



VOLTAIC ELECTRICITY, 



285 



with a glowing spark into the test tube, whereupon the splinter will 
kindle into a flame. 

(c.) Each coulomb of electricity liberates 0. 1176 cu, cm, of hydrogen 
and 0.0588 cu. cm. of oxygen, or a total of 0.1764 cu, cm. of the 
mixed gases. The electrolysis of 9 g. of water requires 95,050 
coulombs. 



411. Ions. — The products of electrolysis, like the oxy- 
gen and hydrogen, are called ions; the one that goes to 
the + electrode (or anode) is called the anion; the one 
that goes to the — electrode (kathode or cathode) is called 
the hathion or cathion, 

(a.) The amount of chemical action in a cell is proportional to the 
strength of current while it passes. One coulomb of electricity, in 
passing through a cell, liberates 0.0000105 gram of hydrogen and 
dissolves 0.00034125 gram of zinc. 

(&.) One coulomb will cause the deposition of 0.0003307 gram of 
copper. Tq deposit 1 gram of copper requires 3024 coulombs. This 
principle has been used in the Edison meter for electric lighting 
purposes, a certain proportion of the current being shunted through 
a " copper voltameter " or bath of copper sulphate solution, as de- 
scribed in the next experiment. 

Experiment 78.— From the + pole of a voltaic battery or dy- 
namo-electric machine, suspend a plate of copper ; from the — pole, 




Fig. 195. 

suspend a silver coin. Place the copper and silver electrodes in a 
strong solution of copper sulphate (blue vitriol). When the circuit 



386 VOLTAIC ELECTRICITY, 

is closed, the salt of copper is electrolyzed, the copper from the salt 
being deposited upon the silver coin and the sulphuric acid going to 
the copper or + electrode. The silver is thus electro-plated with 
copper. (Fig. 195.) 



413. Electro-Metallurgy. — The many applica- 
tions of this process of depositing a metallic coat on a 
body prepared for its reception, constitute the important 
art of electro-metallurgy. If, with the apparatus used in 
the last experiment, a solution of some silver salt be used 
instead of the copper sulphate solution and the direction 
of the current be reversed, silver will be deposited upon 
the copper plate, which will thus be silver-plated. If the 
positive electrode be a plate of gold and the bath a solu- 
tion of some salt of gold (cyanide of gold dissolved in a 
solution of cyanide of potassium), gold will be deposited 
upon the copper of the negative electrode, which will be 
thus electro-gilded. In electrotyping^ impressions of type 
or engravings are taken in wax, or any other plastic ma- 
terial that is impervious to water. A conducting surface 
is given to such a mould by brushing finely powdered 
graphite over it ; it is then placed in a solution of sulphate 
of copper facing a copper plate. The mould is then con- 
nected with the — poleof a dynamo or a voltaic battery and 
the copper, with the + pole; when the current passes 
through the bath, copper will be deposited upon the mould. 
When the copper film is thick enough (say as thick as an 
ordinary visiting card), it is removed from the mould and 
strengthened by filling up its back with melted type- 
metal. The copper film and the type-metal are made to 
adhere by means of an amalgam of equal parts of tin and 
lead. The copper-faced plate thus produced is an exact 



VOLTAIC ELECTRICITY. 287 

reproduction of the type and engravings from which the 
mould was made. 

{a.) In all these cases, the metal is carried in the direction of the 
current and deposited upon the negative electrode. In electro- 
plating and gilding, the technicalities of the art refer chiefly to the 
means of making the deposit firmly adherent. In electrotyping, 
they refer chiefly to the preparation of the mould or matrix. 

413. Electro -Chemical Series. — The facts just 
considered suggest a division of substances into two 
classes^ electro-positive and electro-negative. Tl%e ion 
that goes to the negative electrode is called electro- 
positive; that which goes to the positive electrode 
is called electro-negative, 

(a.) Katliions are called electTO-positive because they seem to be 
attracted to the negative pole of the battery (kathode), the idea be- 
ing that of attraction between opposite electricities. Hydrogen and 
the metals are kathions or electro- positive. They seem to move with 
the current, going as far as possible and being deposited where the 
current leaves the "bath" or electrolytic cell. Similarly, anions 
are said to be electro-negative. 

414. TheE. M. F. of Polarization.— The prod- 
ucts of electrolysis have a tendency to reunite by virtue 
of their chemical aflBnity. (CJiemistry, § 8.) For exam- 
ple, the electrolysis of zinc sulphate gives zinc and sul- 
phuric acid. But we now well know that the chemical 
action of these two substances has an electro-motive force 
of its own. This E. M. F. of the ions acts in opposition 
to that of the electrolyzing current. In some cases, it 
rises higher than the E. M. F. of the original current a7id 
reverses the direction of the current. The oxygen and 
hydrogen, yielded by the electrolysis of water, tend to re- 
unite and set up an opposing E. M. F. of about 1.45 volts. 



388 VOLTAIC ELECTRICITY, 

Thus we see that it requires a battery or cell with an E. 
M. F. of more than 1.45 volts to decompose water. This 
electro'inotive force of the ions is called the E, M, 
F. of Polarization, It may be observed by putting a 
galvanometer in the place of the battery of the water- 
voltameter (Fig. 194). The polarization in a voltaic cell 
acts in the same way. 

{a.) There is no opposing E. M. F. of polarization when the kathion 
and the anode are of the same metal. For example, the feeblest 
current will deposit copper from a solution of copper sulphate, when 
the anode is a copper plate. 

Experiment 79. — Suspend two strips of bright sheet lead facing 
each other in dilute sulphuric acid. Pass a current through these 
plates by connecting them with a battery of 4 or 5 cells in series. A 
dark peroxide of lead will form on one of the bright plates. Then 
remove the battery and, in its place, put a short coil galvanometer or 
electro-magnet. It will be found that the lead-plate cell is supply- 
ing a current, the direction of which is the reverse of the charging 
battery previously used. 

415. Secondary Batteries. — When a voltameter 
or an electro-plating bath is supplying a current of elec- 
tricity, as mentioned in the last paragraph^ it constitutes 
a secondary battery. As the ions do not reunite when the 
circuit is open^ the energy of the decomposing current 
may be stored up as energy of chemical affinity. When 
a current is again wanted, the circuit r}^ay be 
closed and the energy of chemical affinity at once 
appears as energy of electric current. Secondary 
batteries are, consequently, often called storage 
batteries, 

(a.) The Faure battery consists of two plates of sheet lead coated 
with red lead (lead sesqui-oxide, Pb3 04). These plates are sepa- 



VOLTAIC ELECTRICITY, 



289 



rated by a layer of paper or cloth, rolled up in a loose coil like a roll 
of carpet and immersed in dilute sulphuric acid. 

ih.) When a current from a dynamo-electric machine or a voltaic 
battery is sent through such a cell, chemical action is produced. 
Oxygen acts on the coating of the anode plate and converts it into a 
higher oxide of lead (the peroxide, PbOg). Hydrogen acts upon 
the coating of the kathode plate and reduces it to metallic lead in a 
spongy condition. When these changes have gone as far ^s possi- 
ble, the battery is said to be '' charged." The charged plates will 
remain in this condition for days if the circuit he left open. 

(c) By closing the circuit, the plates will, at any time, furnish a 
current until they are changed to their original chemical condition. 
As the lead plates and the acid are not rapidly destroyed, the battery 
may be charged and discharged many times. 




Fig. ig6. 

(d.) Many serious defects in the Faure battery have been obviated 
in the Brush battery (Fig. 196). These batteries are composed of a 
number of cells containing cast lead plates of a peculiar construction, 
electro-chemically prepared and immersed in dilute sulphuric acid. 
These cells may be connected together, tandem or abreast, so as to 
produce any desired result. A large number of these batteries may 
be placed in one circuit and charged by the current of one dynamo. It 
will thus be seen that the dynamo may be made to do double duty, 
charging batteries by day for use in connection with the incandes- 
cence lamps and supplying arc lamps direct, at night. The E. M. F. 



390 



VOLTuHIC JSLECTRICrrr, 



of each Brush cell is about two volts. For electric lighting, they 
are generally prepared in batteries of twenty or more cells. An 
automatic current *' manipulator " or switch is provided with each 
Brush battery and is arranged so as to retain the battery in circuit 

until it is charged and 
then to disconnect it from 
the circuit. When the 
charge lias been exhausted 
to a certain point, it brings 
the battery into the cir- 
cuit again and holds it till 
it has been recharged and 
then cuts it out as before. 
The same operation is re- 
peated with every battery 
in circuit. The operation 
is automatic. Each bat- 
tery has a clock attached, 
which registers the time 
that the charging current 
has been passing through 
Fig- ^97. the cells. The incandes- 

cence lamps are connected with the batteries through the " manipu- 
lator," as shown in Fig. 197. The quantity of electricity capable 
of being " stored " may be increased by increasing the number of 
cells and the size of the plates. 




416. Magrnetic Effects of the Electric Cur- 
rent.— Any conductor is rendered magnetic by passing 
a current of electricity through it. A common needle 
may be magnetized by winding about it an insulated cop- 
per wire and discharging a Ley den jar through the wire. 
We have already seen that a bar of soft iron may be tem- 
porarily magnetized by the influence of the voltaic current. 
It may be further shown by the action of the bar and 
helix. 



{a) This apparatus consists of a movable bar of soft iron surrounded 
by a coil of insulated copper wire (Fig. 198). When the wire of the 
coil is placed in the closed circuit of a battery, the iron bar becomes 



VOLTAIC ELECTRICITY, 



291 




Fig. I 



Strongly magnetized ; when the circuit is broken, the bar instantly 

loses its magnetic power. The bar may be a 

straight piece of stout iron wire ; the helix may 

be made by winding insulated coj^per wire upon 

a piece of glass tubing large enough to admit 

the wire and not quite as long as the iron. 

(&.) A good helix, convenient for many pur- 
poses, may be made upon an ordinary wooden 

spool. With a sharp knife, make the shank of 

the spool as thin as possible and then wind the 

spool full of insulated copper wire about as large as ordinary broom 

or stove-pipe wire. The iron bar must be small enough to pass 

easily through the hole in the spool and long enough to project a 

little ways beyond eacli end. 

(c.) Either of these helices may be placed in the circuit of a cell 

and held in a vertical position, when it will act as a ''sucking" 

magnet. The movable iron core will be held in mid-air '* without 

any visible means of support." 

((7.) The *' helix and ring armature " is shown in Fig. 199. The 
armature is of soft iron divided into two semicircles 
with brass handles. When the helix is placed in a 
closed circuit, the semicircles resist a considerable 
force tending to draw them apart ; when the circuit 
is broken, they fall asunder of their own weight. 
The iron ring may be made without handles by any 
blacksmith. Stout cords will answer for handles. 
The helix may be made by winding insulated wire 
upon a pasteboard cylinder an inch or an inch and a 
half long There should be four or five layers of 
stout, copper wire which may be tied together with 

strings passing through the hole in the helix. 

(^.) Such temporary magnets as these are called electro-magnets. 

The subject of electro-magnets will be further considered in §§ 442- 

448. 




Fig. 199. 



417. Deflection of the Magnetic Needle.— 

We have already seen that the voltaic current has a 
marked effect in turning the magnetic needle from its north 
and south position, tending to place the needle at right 
angles to the direction of the current. This may be easily 
shown by Oersted^s apparatus represented in Fig. 200. It 




292 VOLTAIC ELECTRICITY, 

consists of a magnetic needle and a brass wire frame with 
three pole-cups, permitting the current to be passed over, 

under, or around the magnet. The 
space irmnediately surround- 
ing a ivire carrying an electric 
current is a field of magnetic 
force as truly as is the space 
around a magnetized body 
(§ 433). 

Fig. 200. {a.) If the current pass a&6>^e the needle 

from north to south, the north-seeking or 
— end of the magnet will be deflected toward the east ; if it pass 
from south to north, the — end of the needle wiU be deflected toward 
the west. If the current pass helow the needle, the deflections will 
be the opposite of those just mentioned. The wires are insulated 
where they cross at a. 

418. The Astatic Galvanometer. — This gal- 
vanometer depends upon the principles set forth in the 
last paragraph. It is a very delicate instrument for 
detecting the presence of an electric current and 
deterjnining its direction and strength. In Oersted's 
apparatus, the needle is heavy and a considerable force is 
needed to set it in motion ; in the galvanometer, the needle 
is very light and suspended so as to turn easily. In Oersted's 
apparatus, the needle is held in the magnetic meridian by 
the directive influence of the earth ; in the galvanometer, 
this is obviated almost wholly by the use of an astatic needle 
(§ 439). In Oersted's apparatus, the current makes but a 
single course about the needle ; in the galvanometer, the 
wire is insulated and coiled many times about the needle; 
thus the effect is multiplied. One of the needles is within 
the coil while the other swings above it, the two being 
connected by a vertical axis passing through an appro- 



VOLTAIC ELECTRICITY, 



293 




Fig. 201. 



priate slit in the coil. If both needles were within the 
coil, since their poles are reversed, the same current would 
tend to deflect them in opposite directions and thus the 
action of one needle would neutralize 
that of the other. The astatic needle 
is suspended by an untwisted silk 
fibre from a hook which may be low- 
ered when the instrument is not in 
use until the upper needle rests upon 
the dial plate beneath it. The ends 
of the coiled wire are connected with 
binding screws ; leveling screws are 
provided, by means of which the in- 
strument may be adjusted so that the 
needles shall swing clear of all obstructions. A glass 
cover protects from dust and disturbance by air currents. 
The instrument is represented in Fig. 201. 

(a.) When tlie deflections of tlie astatic galvanometer are less than 
10° or 15°, they are very nearly proportional to the strengths of the 
currents that produce said deflections. A current that deflects the 
needle 6^ is about three times as strong as one that deflects it 2°. 

(6.) That a galvanometer shall be good, it must be able to meas- 
ure the strength of the current in some certain way. It must be 
adapted to the currents to be measured by it. A galvanometer fitted 
for the measurement of small currents {e, g., five or six milliamperes) 
would not be suitable for measuring a ten ampere arc electric 
light current. If the current to be measured has passed through 
a circuit of great resistance {e. g., several miles of telegraph wire), 
a short-coil galvanometer consisting of only a few turns of wire will 
not answer ; a long-coil galvanometer, with many turns of wire 
about the needle, must be used. Hence, it will be seen that differ- 
ent kinds of galvanometers are needed for different kinds of work. 
(See Appendix L. ) 

Experiment 80. — Connect an iron and a German silver wire to 
the binding posts of a sensitive, short-coil, astatic galvanometer. 
Twist the free ends of the wires together and heat the junction in 



394 



THERMO-ELECTRICITY. 



tlie flame of an alcohol lamp. The deflection of the galvanometer- 
needle will show that an electric current is traversing the circuit. 
Cool the junction with a piece of ice. The galvanometer will show 
that a second current is flowing in the opposite direction. 



419. Thermo-Electricity. — // a circuit be 
made of two metals and one of the junctions be 
heated or chilled, a current of electricity is fjro- 
diiced, 

{a.) This may be further illustrated by the apparatus shown 

in Fiff. 202. The 
^ upper bar, m n, 

having its ends 
bent, is made of 
copper ; the low 
er, op, is of bis- 
muth. This rect- 
angular frame is 
to be placed in the 
magnetic merid- 
ian and a mag- 
netic needle 
placed within it. 
Upon heating one 
of the junctions, 

a current wall be produced, the existence of which is satisfactorily 
shown by the deflection of the needle as indicated in the figure. The 
junction may be chilled with a piece of ice or by placing upon it 
some cotton wool moistened with ether. In this case, a current, 
opposite in direction to the first, will be produced ; the needle will 
be turned the other way. The frame may be simplified by bend- 
ing a strip of copper twice at right angles to make the top, bottom 
and one end of the frame, the other end being a cylinder of bis- 
muth. But the form shown in Fig. 202 is preferable, as the same 
junction may be heated by the lamp below or chilled by laying a 
piece of ice on the upper side. 

420. A Thermo-electric Pair.— If a bar of 

antimony, A, be soldered to a bar of bismuth, B, and the 
free ends joined by a wire, we evidently have a circuit 




Fig. 202. 



THERMO-ELECTRICTTY. 295 

equiyalent to the one considered in the last paragraph. 
When the junction, (7, is heated, a current will pass, from 
bismuth to antimony across the junction and from anti- 
mony to bismuth through the wire, as shown in Fig. 203. 

ia.) The arrangement is analogous to a voltaic element, the 
antimony representing the — plate and __^ 

carrying the 4- electrode, the bismuth rep- A 

resenting the + plate and carrying the — C^^^~^^~" 
electrode, while the solder takes the place B 

of the liquid. The E. M. F. of an antimony- yig. 203. 

bismuth pair for 1° C. difference of temper- 
ature is about 117 microvolts. Just as a number of voltaic elements 
may be connected, so may a number of thermo-electric pairs be 
connected to form a thermo-electric series. 



431. The Thermo-electric Pile. — Several 
thermo-electric pairs, generally five, six, or seven, are 
arranged in a vertical series, as shown in Fig. 204, the 
intervening spaces being much reduced, the successive 
bars separated by strips of varnished paper only and the 
wire connection omitted. A similar series may be united 
to this by soldering the free end of the antimony bar of 
one series to the free end of the bis- 
muth bar of the other, the two series 
being separated by a strip of varn- 
ished paper. Any desirable number 
of such series may be thus united, 
compactly insulated and set in a 
metal frame so that only the sold- 
ered ends are open to view. The free end of the antimony 
bar, representing the + electrode, and the free end of 
the bismuth bar, representing the —electrode, are con- 
nected with binding screws, which may be connected with 
a sensitive short-coil galvanometer. The thermo-electric 




296 



THERMO-ELECTRICITY, 



pile, with the addition of conical reflectors, is shown 
in Fig. 205. A change of temperature at either exposed 
face of the pile produces a feeble current of electricity 
which is manifested by the movement of the needle of the 

galvanometer. The instrument 
is much used in scientific work 
for detecting differences in tem- 
perature, being much more 
sensitive than the mercury ther- 
mometer. 




Fig. 205. 



433. The Peltier Ef- 
fect. — When an electric cur- 
rent passes over a junction 
from antimony to bismuth, 
there is an evolution of heat at 
the junction, the temperature 
of which rises. When the current passes in the op- 
posite direction (from bismuth to antimony), there is an 
absorption of heat and the temperature of the junction 
falls. In other words, if the current be sent through the 
circuit in the direction in which the thermo-electromotive 
force would naturally send it, the heated junctions will be 
cooled and the cooled junctions will be heated. 



Exercises. 

1. {a.) Draw a figure of a simple voltaic element, (b.) State 
what is meant by the electric current, (c.) Indicate, upon the 
figure, the direction of the current, {d.) What are the electrodes ? 
{e.) Indicate them by their proper signs upon the figure. 

2. {a.) Describe or figure a high resistance battery of Grove's ele- 
ments. (&.) A low resistance battery of Bunsen's elements, (c.) 
What is the peculiar advantage of the Daniel Fs battery ? 



VOLTAIC AND THERMO-ELECTRICITY. 297 

3. Describe an experiment illustrating the heating effects of cur- 
rent electricity. 

4. {a.) How may a very feeble current be detected ? ih.) Describe 
the apparatus used, (c.) Mention the features contributing to its 
delicacy. 

5. {a.) If the resistance of one mile of a certain electric light wire 
is 3.58 ohms, what is the resistance of 4.4 miles of the same wire? 
(5.) The resistance of a certain wire is 5 ohms per 100 yd. What 
length of the same wire will have a resistance of 13.2 ohms? 

Ans. (a.) 15.75 ohms. (6.) 264 yd. 

6. What is the resistance of a mile of copper wire that has a 
diameter of 65 mils if the resistance of a mile of copper wire 80 mils 
in diameter is 8.29 ohms ? Ans. 12.56 ohms. 

7. If the resistance of 700 yd. of a certain wire is 0.91 ohm, what 
is the resistance of 1,320 yd.? Ans. 1.72 ohm. 

8. (a.) Define electrolyte. (&.) What term is applied to chemical 
decomposition when effected by means of an electric current ? (c) 
How would you go about the task of determining for yourself the 
electro-chemical nature of a substance ? 

9. The resistance of a certain wire is 4.55 ohms. The resistance 
of a mile of the same wire is 1.3 ohms. What is the length of the 
first wire? Ans. 3.5 mi. 

10. The resistance of a mile of copper wire 70 mils in diameter is 
10.82 ohms. What is the diameter of a copper wire a mile long and 
having a resistance of 23 ohms ? Ans. 0.048 inch or 48 mils. 

11. What should be the length of a silver wire so that it may 
have the same resistance as 10 inches of copper wire of the same 
thickness, the conductivity of silver being 1.0467 times that of 
copper? 

12. Find the resistance, at the freezing temperature, of 20 m. of 
German silver wire weighing 52.5 grams, having given that the resist- 
ance, at the same temperature, of a wire of the same material 1 m. 
long and weighing 1 g. is 1.85 ohms. Ans. 14 1 ohm. 

13. When a piece of fine platinum wire and a galvanometer are 
put in the circuit of a galvanic cell, the needle is deflected. Remove 
the platinum wire and close the circuit with stout copper wire ; 
the needle is deflected more than before. Explain. 

14. Find the resistance of 500 yd. of copper wire 165 mils in 
diameter, the resistance of one mile of copper wire 230 mils in 
diameter being one ohm. Ans. 0.55 ohm. 

15. If 1,000 ft. of wire 95 mils in diameter have a resistance of 
1.15 ohm, what is the diameter of a wire of the same material that 
has a resistance of 10.09 ohms per 1,000 ft.? Ans. 32 mils. 



298 



VOLTAIC AND TBERMO-ELECTRICITY. 




\ 



16. Under wliat circumstances is it desirable to arrange cells 
as shown in Fig. 206 ? 

17. A copper wire 6 m. long has a diameter of 0.74 mm. What 
is the length of a copper wire of 1 mm. diameter that has the same 

electrical resistance ? Ans. 10.957 m. 

18. Given 8 cells, each with an E. M. F. of 2 volts 
and an internal resistance of 8 ohms. The resistance 
of the external circuit is to be 16 ohms. How shall 
the cells be arranged to give maximum current and 
what will that current be? Ans, 0.25 ampere 

19. What is the length of an iron wire having a 
sectional area of 4 sq. mm. and the same resistance as 
a copper wire 1,000 yd. long, the latter having a sec- 
tional area of 1 sq. mm., the conductivity of iron 
being \ that of copper ? Ans. 571 f yd. 

20. Two incandescence lamps of 31 and 37 ohms 
respectively are placed abreast in a circuit. Find the 
joint resistance of the two lamps. Ans, 16.87 ohms. 

21. How thick must an iron wire be so that it and 
a copper wire that has the same length and a diame- 
ter of 2.5 mm. shall have the same resistance, the re- 
sistaDce of iron being 7 times that of copper? 

Ans. 6.61 mm. 

22. How many coulombs will be furnished by the consumption of 
20^. of zinc? 

23. What weight of zinc must be consumed in each cell of a 
voltaic battery of 3 Daniell's cells to enable the electrolysis of 9 g, 
of water? (Neglect loss by local action.) Ans. About 32.5 g. 

24. What weight of copper will be deposited in each cell of the 
battery mentioned in the last problem? Ans. About 31.5 g. 

25. Three wires, the respective resistances of which are 5, 7 and 
9 ohms are joined in multiple arc. Find the resultant resistance of 
this compound conductor. Ans. 2.2 ohms. 

26. What is the necessary E. M. F. of a dynamo that is to furnish 
a 10 ampere current for 60 arc lamps (in series), each of which has a 
resistance of 4.5 ohms, the resistance of the line wire being 10 ohms 
and the internal resistance of the dynamo being 22 ohms ? 

27. A piece of zinc, at the lower end of which a piece of copper 
wire is fixed, is suspended in a glass jar containing a solution of 
acetate of lead (sugar of lead). After a few hours, a deposit of lead 
in tree-like form grows downward from the copper wire. Explain 
this. 

28. Liquids increase in conductivity with an increase of temper- 



FlG. 2o6. 



VOLTAIC AND THERMO-ELECTRICITY. 299 

ature. Will a given battery give a stronger current at 0° C. or at 
20° C? 

29. What should be the length of a lead wire so that it may have 
the same resistance as 10 inches of copper wire of the same thickness, 
the conductivity of lead being 0.0923 times that of copper ? 

30. Four wires are joined together in multiple arc, their resist- 
ances being 5.5, 18, 3.7 and 2.9 ohms respectively. Find the result- 
ant resistance of the compound conductor thus formed. 

Ans. 1.17 ohm. 

HOl^ORAEY PROBLEM. 

31. Find the number of incandescence lamps that may be worked 
in multiple arc by a dynamo-electric machine that has an internal 
resistance of 0.032 ohm. The E. M. F. of the dynamo is 55 volts 
and the resistance of each lamp is 28 ohms. The current must be 
1.6 amperes in each lamp. Ans. 199 lamps. 



300 



VOLTAIC AND THERMO-ELECTRICITT. 



Recapitulation. — To be amplified by the pupil for 



reyiew. 






P4 
H 
U 

H 

u 



r VOLTAIC. 



Cell. 



One Liquid. 



Two Liquids. 



Smee^s. 
Potassium 

di-chromate. 
Leclanche. 

Daniell's. 
Callaud's. 
Grove's. 
Bunsen's. 

( Tandem. 

Joined. •< Abreast. 

( Best Method. 
Source of Energy. 



CIRCUIT . 



Battery. 



_ Current.. 

Simple. 

Divided. 

Shunt. 



High Internal Resistance. 
Low Internal Resistance. 
Requisites. 

Direction . xr .. 



( Plate. 

SIGN OF \ Pole. , . ^ .^ 

1 Electrode. \ j^Zt^e. 

POTENTIAL; Fall of E. M. F. -j 



RESISTANCE . 



Unit. 
Measurement. 

External. 

Internal. 

Laws. 

Unit. 

Measurement. 

Long and Short-Coil Instruments. 



QUANTITY; Unit. 

Cause. 
Remedy. 



LOCAL ACTION, -j 

I POLARIZATION. ] g^^f^^^ 

r MECHANICAL. 
THERMAL ; Relation to Resistance. 

\ Electrolysis. ] ^X^^. \ 
Electro-Metallurgy. 
Electro-Chemical Series. 



LUMINOUS. 
PHYSIOLOGICAL. 
CHEMICAL 



Ions. 



MAGNETIC. . 



E. M. F, OF Polariza- 
tion. 

Secondary Batteries.. 



Electro-Magnets. 
Electric Telegraph. 
Galvanometer. 



r Faure^s. 
J Brush's. 
I Uses, 
y Advantages, 



THERMO-ELECTRICITY. 



(J^or Induced Currents.^ see Section V. of this Chapter^ 



^Sec tjon IV. 



MAGNETISM. 

433. Natural Magnets, — One of the most valua- 
ble iron ores is called magnetite (Fcg O4). Occasional 
specimens of magnetite will attract filings and other pieces 
of iron. Such a specimen is called a lodestone. 
It is a natural magnet. 

434. Artificial Magnets. — Artificial magnets are 
either temporary or permanent. A temporary magnet 
is usually made of soft iron and is called an electro- 
magnet. A permanent magnet is usuaHy made of steel. 
Artificial magnets haye all the 
properties of natural magnets 
and are more powerful and con- 
venient. They are, therefore. Fig. 207. 
preferable for general use. The 

most common forms are the straight or lar magnet and 
the horseshoe magnet. The first of these is a straight bar 
of iron or steel; the second is shaped like a letter U, the 
ends being thus brought near together, as shown in Fig. 
207. A piece of iron placed across the two poles of a 
horseshoe magnet is called an armature. We have already 
learned how to make artificial magnets. 

435. Retentivity. — It is more difiicult to get the 
magnetism into steel than into iron. It is also more diffi- 




302 



MAGNETISM. 



cult to get it out. This power of resisting magneti' 
zation or demagnetization is called coercive force 
or retentivity. The harder the steel, the greater its re- 
tentiyity. Soft wrought iron has but Httle retentivity. 

436. Distribution of Magnetism.— If a bar 

magnet be rolled in iron fihngs and then withdrawn, the 




Fig. 208. 



filings cling to the ends of the bar but not to the middle. 
This form of attraction is not evenly distributed through- 
out the bar. It is greatest at or near the ends. 
These points of greatest attraction are called the 
poles of the magnet. It is impossible, by any known 
means, to develop one magnetic pole without simultane- 




MAGNETISM, 303 

ously developing another pole of opposite sign. The mid- 
dle of the magnet does not attract iron and is called the 
equator or neutral point. 

Experiment 31. — Bring either end of a bar magnet near the end 
of a floating piece of iron, AB ; the iron is 
attracted. Bring the same end of the 
magnet near the middle of the iron ; the 
iron is attracted. Bring the same end of 
the magnet near the other end of the iron ; 
the iron is attracted. Repeat the experi- 
ments with the other end of the magnet ; ~ :=■ "" 

in each case, the iron is attracted. Fig. 209. 

437. Attraction between a Magnet and 
Iron. — Either pole of a magnet will attract or- 
dinary iron. 

Experiment 82.— Freely suspend three bar magnets, A, B and (7, 
at some distance from each other. This may be done by placing each 
magnet in a stout paper stirrup supported by a cord or horse-hair or 
upon a board or cork floating on water. (See Fig. 209.) When they have 
come to rest, each will lie in a north and south line. Magnets for this 
experiment may be made by magnetizing (§ 448) three stout knitting- 
needles. If there is any electric light apparatus in your neighbor- 
hood in charge of a good-natured man, he will probably magnetize 
the needles for you. Each needle may be suspended by means of a 
triangular piece of stiff writing-paper. Pass the needle through the 
paper near the lower corners ; at the other corner, affix, by wax, the 
end of a horse-hair. The poles may be indicated by little bits of red 
and of white paper, fastened by means of wax to the ends, of the 
needles. Mark the north-seeking poles, — and the south-seeking 
poles, -F. 

438. Characteristics of Magnets.— Magnets 
are chiefly characterized by the property of attract- 
ing iron and by a tendency to assume a partic- 
ular direction of position when freely suspended. 

Experiment 83. — {a) Take magnet A of Experiment 82 from its 



304 



MAGNETISM, 



support and bring its + end near the — end of B or G. Notice the 
attraction. 

(b.) Bring the + end of A near the + end of B or G* Notice the 
repulsion. 

(c.) Bring the — end of A near the — end of B or G. Notice the 
repulsion. 

{d.) Bring the — end of A near the + end of B or G. Notice the 
attraction. 

[e.) From (a,\ we learned that the — ends of B and G were each 
attracted by the + end of J.. Bring the — end of B near the — 
end of G. Notice that they now repel. 

(/.) From (6.), we learned that the + ends of B and (7 were each 
repelled by the + end of A. Bring the + end of B near the + end 
G. Notice that they now repel. 

{g,) In similar manner, show that the + end of B will attract the 
— end of G ; that the — end of B will attract the + end of G. 

Record the results of your experiments in tabular form thus : 



(a.) -f attracts —. 


(b.) + repels 


{d.) — attracts +. 


(c.) — repels 


etc. 


etc. 



Experiment 84. — Magnetize a number of fine sewing-needles by 
drawing the 4- end of a bar magnet three or four times from the eye 

to the point of each. 
Cut several small 
corks into slices 
about an eighth of 
an inch thick. 
Through each cork 
disc, push a needle 
up to its eye and 
place them in a 
round dish of water. 
These little mag- 
nets have their like 
poles presented to 
Bring the bar magnet, with its 
they will be driven to the 




Fig. 2IO. 



each other and they mutually repel 

+ end downward, over the needles ; 

sides. Similarly, bring the — end over them ; they will be attracted 

toward the centre. 



4:39. Laws of Magnets.— (1.) Every magnet 



MAGNETISM. 305 

has two similar poles; like poles repel each other ; 
unlike poles attract each other. 

(2.) Magnetic force, like other forms of attrac- 
tion and repulsion, varies inversely as the square 
of the distance. 

Experiment 85. — Dip one of the magnetized knitting-needles into 
iron filings. Notice that filings cling to the ends, near the paper 
discs, but that none cling to the middle. Break the needle in the 
middle and dip each piece into iron filings. Notice that the un- 
marked ends, which were at the middle of the unbroken magnet, 
now attract iron filings as well as do the marked ends. Poles ham 
teen developed in parts of the needle thojt previously showed no mag- 
netic attraction. 

430. Effect of Breaking a Magnet.— If a 

magnet be broken, each piece becomes a magnet with two 
poles and an equator of its own. These pieces may be 
repeatedly subdiyided and each fragment will be a perfect 
magnet. 




Fig. 211. 

It is probable that every molecule has its poles 
or is polarized and that, could one be isolated, it 
would be a perfect magnet. We may, thus, conceive 
a magnet as made up of molecules each of which is a 
magnet, the action of the molar magnet being due to the 
combined action of all the molecular magnets of which it 
is composed. 

431. Magnetized, Magnetic and Diamag- 
netic Substances. — A magnetized body is one that 



306 MAGNETISM, 

can be made to repel a pole of a freely suspended magnet. 
Substances that are attracted by a magnet are called 7nag- 
netic; e,g,, iron or steel and nickel. Substances that are 
repelled by a magnet 2i>vQ QdiW^di diamagiietic ; e,g.y bismuth, 
antimony^ zinc, tin, mercury, lead^ silver, copper, gold 
and arsenic. Of these, iron is by far the most magnetic, 
while bismuth is the most diamagnetic. The magnetic 
properties of iron or steel are easily shown ; diamagnetic 
properties require a powerful magnet for satisfactory illus- 
tration. 

Experiment 86. — Wrap a bar magnet in a piece of cloth. With 
it, attract and repel the poles of a suspended magnet. 

Experiment 87. — Repeat the last experiment, holding a slate or 
sheet of zinc between the two magnets. 

Experiment 88. — Put one piece of the broken magnet into a bot- 
tle ; cork the bottle tightly. With it, attract and repel the poles of 
a suspended magnet. 

433. Magnetic Screens. — Jfothing hut a mag- 
netic body can cut off the inductive action of a 
magnet. If a small magnet be suspended inside a hol- 
low iron ball, no outside magnet will affect it. 

Experiment 89. — With the end of a good bar magnet, write your 
name upon the blade of a handsaw. The invisible characters may 
be made visible by sifting fine iron filings upon the blade. 

Experiment 90. — Place a piece of card-board or rough drawing 
paper over a good bar magnet. Sift fine iron filings through a piece 
of muslin upon the card-board and tap it lightly. The iron particles 
will move and arrange themselves in well defined curved lines. (See 
Fig. 212.) By using two bar magnets placed side by side, first, with 
like poles near each other and, secondly, with unlike poles near each 
other, their combined effect on the iron filings may be easily ob- 
served. The figures will be widely different. 



MAGNETISM. 



307 



433. Magnetic Field. — A magnet seems to be sur- 
rounded by an atmosphere of magnetic influence called 
the magnetic field. (See § 450^? and Appendix N.) 
The magnetic curves^ formed in the above experiment, 
are very interesting and instructive for they show the 
direction of the lines of magnetic force. The filings 
in any one of these curves are temporary magnets with 





Fig. 212. 



adjoining poles opposite and therefore attracting. If a 
small magnetic needle be suspended over the card board 
at any point, its length will tend to lie in the direction 
of the lines of magnetic force at that point as mapped out 
by the iron filings. 

(«.) The fio:ures may be permanently fixed by using a sheet of 
glass that has been gummed and dried, instead of the sheet of 
paper. The filings are sifted evenly over the surface ; then the glass 
is tapped ; then a jet of steam is caused to play gently above the 
sheet, softening the surface of the gum, which, as it hardens, fixes 
the filings in their places. 

(6.) Since the lines of force are made of little magnetic particles 
that set themselves thus in obedience to the attractions and re- 
pulsions in the field, they represent the resultant direction of said 
forces at each point. They map out the magnetic field, shovnng the 
direction of the magnetic force by their position and its intensity by 



308 MAGNETISM, 

their number. If a small — pole could be obtained alone and put 
down on any one of these lines of force, it would tend to move along 
that line from + to — ; a single + pole would tend to move along 
the line in an opposite direction. 

Experiment 91. — Rub one end of a steel pen against the end of a 
magnet. Dip the pen into iron filings and notice that the newly 
made magnet has a pole at each end. Determine the sign of each of 
these poles, as indicated in Experiment 82. 

434. Magnetization by Contact. — A har of 

iron or steel may he magnetized by rubbing it 
against a magnet. Pure or soft iron is easily magnet- 
ized but quickly loses its magnetism when the magnetiz- 
ing influence is removed. Hardened steel is magnetized 
with more difficulty but retains its magnetism after the 
removal of the magnetizing influence. 

Experiment 92. — Move the point of an nnmagnetized steel pen to 
and fro very near one end of a magnet but without touching it to 
the magnet. Dip the pen into iron filings and determine whether 
or not it has been magnetized. Tf it has, determine the sign of each 
pole, as in the last experiment and notice whether the point of the 
pen is of the same polarity as the end of the magnet near which it 
was moved. 

Experiment 93. — Bring a short bar of soft iron, /, very near a 
strong bar magnet, M, end to end, as shown in the figure. Sprinkle 




Fig. 213. 

iron filings over the ends of the iron bar and they will cling as they 
would to a magnet. The iron har is a magnet^ while it remains in 
this position. 

435. Magnetic Induction. — If the end of a bar 

of soft iron be brought near one of the poles of a strong 



MAGNETISM, 309 

magnet, the iron becomes, for the time being, a mag- 
net. The poles of the temporary magnet will be opposite 
to those of the permanent magnet;, i.e., if the + or posi- 
tive pole of the magnet be presented to the iron bar, it 
will develop a — or negative pole in the nearest end of the 
iron bar and a + pole at the further end. Bring the iron 
bar nearer the magnet and this eflfect will be increased. 
Actual contact is not necessary, but when the iron and 
the magnet touch, the magnetizing force is the greatest. 
If a steel bar be used instead of an iron bar, it will be per- 
manently instead of temporarily magnetized. The iron 
or the steel is induced to become a magnet by the 
influence of the magnet used. It is said to be 
m'agnetized by induction. This, like other forms of 
attraction, varies inversely as the square of the distance. 
We have already seen that magnetic induction takes place 
in certain directions called lines of magnetic force 
(§ 433.) 

Experiment 94. — Bring a soft iron ring to the end of a magnet. 
It will be supported. Bring a second ring into contact with the 
first ring and it will be sup- 
ported. In this way, quite 
a number of rings may be 
supported, each ring being 
magnetized by the bar or 

ring magnet above it. Of Fig. 214. 

course, the attractive force 

is continually weakening from the first to the last ring. Sup- 
port the upper ring upon your finger and remove the magnet. 
Each ring ceases to be a magnet and the chain is broken into 
its separate links. Vary the experiment by using, instead of 
the rings : (1,) Soft iron nails ; (2,) Steel sewing- needles ; and 
see if there is any difference in the results. 

Experiment 95. — Suspend an iron key from the positive end of 
a bar magnet. The key is inductively magnetized, the relation of 




310 



MAGNETISM. 



its poles to each other and to the magnet being as shown in Fig. 
215. A second bar magnet of about the same power, with its poles 
opposite, is moved along the first magnet. When the — end of the 
second magnet comes over the key, the key dro'ps. 




Fig. 215. 



The first magnet tends to induce a - pole at the upper end of the 
key. The second magnet tends to induce a + pole at the same 
point. The effect of each magnet neutralizes that of the other. 

Experiment 96. — Magnetize a piece of watch spring about six 
inches long (easily obtainable at the watch repairer's) by drawing it 
several times between the thumb and the end of a magnet. Dip it 
into iron filings. Lift it carefully with its load. Bring the poles of 
the spring magnet together, bending the magnet into a ring. The 
magnet drops its load. 

436. Induction Precedes At- 
traction. — We now see why a magnet 
attracts ordinary iron; it first magnetizes 
it and then attracts it. The attraction be- 
tween unlike poles is greater than the re- 
pulsion between like poles because of the 
smaller distance between them. Compare 
§336. 

Experiment 97. — Test a common fire poker for 

magnetism by bringing a small magnetic needle near 

its ends and seeing whether the poker repels either 

pole of the compass needle or whether the two ends of the poker 

attract different poles of the needle. If the poker is not even slightly 

magnetic, place it with its upper end sloping toward the south so as 



MAGNETISM, 311 

to make an angle of a little less than half a right angle. In other 
words, place it in the position assumed by the dipping needle. 
(§ 439.) While the poker is in this position, strike it a few blows 
with a wooden block or mallet. Test it again for magnetism. A 
steel poker that has usually stood in a nearly vertical position may, 
thus, often be shown to have acquired magnetism. 

437. The Earth is a Magnet.— The earth acts 
like a huge magnet in determining the direction of com- 
pass and dipping needles. Its inductive influence, as 
shown in the last experiment, strengthens the belief that 
it has such action. If a small dipping needle be placed 
over the — end of a bar magnet, the needle will take a 
vertical position with its + end down. As the needle is 
moved toward the other end of the bar, it turns from its 




Fig. 217. 

vertical position. When over the neutral line, the needle 
is horizontal. As it approaches the + end of the magnet, 
the needle again becomes vertical, but the — end of the 
needle is drawn down. If a dipping needle be carried 
from far southern to far northern latitudes, it will act in a 
similar way. Many facts seem to teach that the earth is 
a great magnet with magnetic poles near its geo- 
graphical poles. The magnetic pole in the northern 
hemisphere was found in 1832 by Capt. Eoss. It was then 
a little north and west of Hudson's Bay, in latitude 



312 MAGNETISM. 

W 05' N., and longitude 96° 45' W. A place in the south- 
ern hemisphere has been found where the dipping needle 
is nearly vertical. 

438. Names of Magnetic Poles.— We have now learned 
to regard tlie earth as a huge magnet, with one pole in the northern 
hemisphere and one in the southern. Since unlike poles attract 
each other, it follows that tlie earth's magnetic 'pole situated in the 
northern hemisphere is opposite, in kind, to the end of a magnetic needle 
that points to the north. From this fact, great confusion of nomen- 
clature has arisen. We have spoken of the end of the needle that 
points north as — or negative. Following this nomenclature, the 
northern magnetic pole of the earth must be + or positive. But 
popular usage calls the north -seeking end of the needle the north 
pole and the other end the south pole. This introduces great confu- 
sion when we wish to speak of the magnetic poles of the earth. 
The nomenclature that we have adopted obviates this confusion. 

Experiment 98. — Make a horizontal needle of a piece of watch 
spring about six inches long and straightened by drawing it between 
thumb and finger. Heat the middle of the 
needle to redness in a flame and bend it 
double. Bend the ends back into a line 
with each other, as shown in Fig. 218. 
Magnetize each end separately and oppo- 
FiG. 2 1 8. sitely. Wind a waxed thread around the 

short bend at the middle to form a socket 
and balance the needle upon the point of a sewing-needle thrust into 
a cork for support. A little filing, clipping with shears or loading 
with wax may be necessary to make it balance. The needle will 
point north and south. 

Experiment 99. — By means of a fine wire fork, gently lay one of 
the magnetized sewing-needles of Experiment 84 on the surface of 
water. It will float vdthout any cork or similar support and will 
assume a north and south position. It may be considered the needle 
of a small compass. 

439. Magnetic Needles. — A small bar inagnet 
suspended in such a manner as to allow it to 
assume its chosen position is a magnetic needle, 
(See Appendix J.) 




MAGNETISM, 



313 




Fig 219. 



{a.) If it be free to move in a horizontal plane, it is a horizontal 
needle; e. g., the mariner's or the survey- 
or's compass (Fig. 219). It will come to 
rest pointing nearly north and south. 
If the magnet be free to move in a ver- 
tical plane, it constitutes a vertical or 
dipping needle (Fig. 220). Two magnets 
fastened to a common axis but having 
their poles reversed constitute an astatic 
needle (Fig. 221). An astatic needle as- 
sumes no particular direction with respect 
to the earth if the two needles are equally 
magnetized. (§ 418.) 

(p.) Make a dipping needle by thrusting a knitting-needle through 

a cork so that the cork shall be at 
the middle of the needle. Thrust 
through the cork, at right angles 
to the knitting-needle, half a knit- 
ting-needle, or a sewing-needle, 
for an axis. Support the ends of 
the axis upon the edges of two 
glass goblets or other convenient 
objects. Push the knitting-needle 
through the cork so that it will 
balance upon the axis like a scale- 
beam. Magnetize the knitting- 
needle and notice the dip. 

{c) A magnetized sewing- 
needle, suspended near its middle 
(at its centre of gravity) by a fine 
thread or hair or an untwisted 
fibre will serve as a dipping needie. 
It should first be suspended so as to hang + 
horizontal and magnetized afterward. 
A simple form of dipping needle is repre- 
sented in Fig. 222. 

440. Inclination or Dip. — 

The angle that a dipping 
needle makes ivith a hori- 
zontal line is called its in- Fig. 221. 
clination or dip. The angle in question is indicated 




Fig. 220. 




314 



magn:b:tism. 



by the dotted arc of Fig. 222. 




Fig. 222. 
masked by the effect of gravity. 



At the magnetic poles, the 
inclination is 90°; at 
the magnetic equator, 
there is no inclina- 
tion. The inclination 
at any given place is 
not greatly different 
from the latitude of 
that place. 

{a.) Experiments for 
inclination are difficult 
of execution without spe- 
cial apparatus. It is diffi- 
cult to make a needle 
turn about a point ex- 
actly coincident with its 
centre of gravity. In 
rough experiments, there 
is danger that the mag- 
netic effect will be 









OJEAT 0/ 



NORTH • 
STAR 






^ a. 



^s- 



^ tsi. 

r Co 



Fig. 223. 



MAONETtSM. 315 

Experiment 100. — Set two stakes so tliat a string joining them 
will point toward the North Star. The string will run north and 
south or nearly enough so for our purpose. Place a long magnet 
suspended as a needle under or over the string. Looking downward 
at the magnet and the string, it will probably be found that the 
needle and the string do not point in the same direction. The 
North Star may be easily found any evening in the direction indi- 
cated by "The Pointers" of the well known constellation, "The 
Great Dipper." ''The Pointers" are the two stars marked by the 
Greek letters a and ,J in Fig. 223. 

441. Declination or Variation. — The magnetic 
needle, at most places, does not lie in an exact north and 
south line. The angle that the needle mahes ivith 
the geographical meridian is its declination or 
variation. A line drawn through all places where the 
needle points to the true north is called a Line of no Vari- 
ation, Such a line, nearly straight, passes near Cape Hat- 
teras, a little east of Cleveland, Ohio, through Lake Erie and 
Lake Huron. It is now slowly moving westward. At all 
places east of the Line of no Variation, the — end of the 
needle points west of the true north ; at all places west of 
the Line of no Variation, the variation is easterly. The 
further a place is from this line, the greater the declina- 
tion, it being 18° in Maine and more than 20° in Oregon. 

{a.) In order that ships may steer safely by the compass, magnetic 
charts are prepared. The declination at various places is properly 
indicated on the chart. The surveyor must recognize not only the 
declination of his needle but also the changes in declination. Other- 
wise he would not be able properly to " run ^l\5~o'j5MWMlfi 
the lines " of a given piece of land from the 
description given in an old deed. 






Experiment 101. — Construct a floating 
cell of zinc and copper plates, about \ inch 
apart, the connecting wire being given an ^ ^ 

elongated spiral or solenoid form, and support ^^^- 224. 

it by a large, flat cork resting on the surface of a bowlful of acidu- 



316 



MAGNETISM, 



lated water, as shown in Fig. 224. The solenoid may be made by 
winding the middle part of about 3 yards of No. 20 insulated copper 
wire around a rod, half an inch in diameter, forming thus a coil, 4 
or 5 inches long. The current will set the axis of the solenoid in a 
north and south direction as if it were a magnetic needle. By hold- 
ing one end of a bar magnet near first one end and then the other 
end of the solenoid, it will be found that the latter exhibits magnetic 
polarity. 




Fig. 225. 



Experiment 102. — Support a solenoid by placing the extremities 
of its wire (bent into the same vertical axis) in two mercury cups, as 
shown in Fig. 225, or use the solenoid of the floating battery above 
described. Bring the end of a second solenoid successively to the 
ends of the first and notice the exhibition of magnetic polarity. 

Experiment 103. — Send a current of electricity from the small 
cell, mentioned in Experiment 16, through its wire. Pour half a 
teaspoon ful of iron filings upon a sheet of paper and bring the wire 
conductor of the cell into contact with the filings. Notice that the 
filings cling to the wire as though it were a magnet. Break the 
circuit and notice that the filings fall from the wire. 

443. Electro-Magnets. — From these experiments, 
we see that while the wire conductor is carrying an electric 
current it has the properties of a magnet. We have 
already seen that, under similar circumstances, the con- 
ductoi' deflects a magnetic needle as if it were itself a 



MAGNETISM, 



317 




Fig. 226. 



magnet. In fact^ such a conductor is a temporary mag- 
net. The magnetic effect is much increased if a con- 
siderable length of the conductor be 
made of msulated wire and wound 
into a coil^ as shown in Fig. 236. 
Such a coil is called a helix; it is a 
magnet with a + pole at one end and 
a — pole at the other. It has an 
easily perceptible magnetic field. If 
a soft iron rod or core be introduced 
into the coil, it enters the magnetic 
field of the coil or helix and becomes 
a magnet. This combination of coil and core consti- 
tutes an electro-magnet and is more powerfully mag- 
netic than the coil alone. An electro-magnet is a 
har of iron surrounded hy a coil of insulated wire 
candying a current of electricity. It may be made 
more powerful than any permanent magnet but loses its 

power as soon as the current 
ceases to flow through its coil. 
The fact that the magnetism of 
this apparatus is under control 
adapts it to many important uses, 
such as electric bells and tele- 
graphic instruments. 

443. Forms of Electro- 
Magnets. — The bar of § 416, a, 
^^''' ^^7. and the ring of Fig. 199, with 

their helices, are electro-magnets. The electro-magnet 
more often has the horse-shoe form shown in Fig. 227, so 
that the attraction of both poles may act upon the same 




318 



MAGNETISM, 



body at the same time. The middle of the bent bar is 
bare^ the direction of the windings on the ends being such 
that^ were the bar straightened^ the current would move 
in the same direction round every part. More frequently, 
the two helices, A and B, have separate cores which are 
joined by a third straight piece into which the ends of the 
cores are screwed. An armature is often placed across the 
two poles of the magnet, as shown in the figure. Electro- 
magnets have been made capable of supporting several 
tons. 

{a.) When the circuit is broken and the current thus interrupted, 
the iron is generally not wholly demagnetized. The small magnet- 
ism remaining is called residual magnetism. The residual magnetism 
seems to increase with the hardness and impurity of the iron. The 
cores of electro-magnets for some purposes are made of the softest 
and purest iron obtainable. 



444. The Electric Telegraph. — The electric 
telegraph consists essentially of an electro-magnet and a 
^^key'' placed in the circuit of a battery. The key is an 

instrument by which 
the circuit may be 
easily broken or closed 
at will. The armature, 
Ay of the '^register' 
magnet, M, is sup- 
ported by a spring, S, 
which lifts it when the 
circuit is broken. 
When the circuit is 
closed, the armature is drawn down by the attraction of 
the magnet. Thus, the armature may be made to vibrate 
up and down at the will of the person at the key. The 




Fig. 228. 



MA GNETtSM. 



319 



armature may act upon one arm of a lever, the other end 
of which, being provided with a style or pencil, P, may be 
pressed against a paper ribbon, R, drawn along by clock- 
work. Thus, the pencil may be made to record, upon the 
moving paper, a series of dots and lines at the pleasure of 
t'le operator at the key perhaps hundreds of miles away. 
When the two stations are several miles apart, one of the 
wires is dispensed with, the circuit being completed by 
connecting each station with the earth. This arrange- 
ment saves half the wire and nearly half the cost of the 
line. As the resistance of the earth is insignificant, there 
is the further saving of nearly half the battery otherwise 
necessary. Earth connections are often made by joining 
the wires to water or gas pipes that run into the ground. 
When the line is long, there is a battery at each end, the 
+ electrode of one battery and the — electrode of the 
other battery being joined to the line wire. The same 
principle of communicating signals by making and break- 
ing an electric circuit is used in fire and burglar alarms, 
hotel annunciators, etc. 

445. Morse's Alphabet. — The inventor of the 
practical electric telegraph was an American, S. F. B. Morse. 
The code of signals devised by him is given below : 



a — 


LETTERS. 
k 


u 


FIGURES. 


h 

c - - - 


I 

m 


w 


2 

3 


d 


n 


X 


4 


e - 

g 

h 


- - 

P 

q 

r - - - 


y - - -- 

z - 

} - - 


6 

8 


i - - 


s 

t — 


f . 


9 





330 



MAGNETISM. 



(a.) To prevent confusion, a small space is left between successive 
letters, a longer one between words and a still longer one between 
sentences. We liere give a sbort message written in Roman and in 
telegrapblc characters ; 



H 



1 1 



ten 



The ordinary telegraph operator does not punctuate his messages 
to any considerable extent. Telegraph operators soon become so 




Fig. 229. 

familiar with this alphabet that they understand a message from the 
mere clicks of the lever and do not use any recording apparatus. 
Such an operator is said to " read by sound " ; his instrument is called 
a "sounder." Fig. 229 represents one. The sounder is placed on a 
local circuit and has a usual resistance of from three to five ohms. 




Fig. 230. 

(&.) With a long main line, the resistance is so great that the cur- 
rent of the main battery is too feeble to operate the sounders with 



MAGNETISM. 



321 



sufficient force. This difficulty is met 
by introducing a " local battery " and 
a " relay " at each station on the line. 
The relay (Fig. 230) is a delicate elec- 
tro-magnet, of which the terminals, 
a and 5, are connected with the main 
line. This magnet operates an ar- 
mature lever, 6, the end of which 
strikes against a metal contact piece 
and thus closes the local circuit 
through the terminals, c and d. The 
resistance of relays vary from 50 to 
500 ohms. The " Western Union " 
standard relay has a resistance of 150 
ohms. 

(c.) The arrangement of instru- 
ments is best studied at a telegraph 
station, one or more of which may be 
found at almost any town or railway 
station. The general features of the 
" plant " are represented by the dia- 
gram shown in Fig. 231. The pupil 
will probably find the key, sounder 
and relay on a table and the local 
battery, 6, under the table. The keys 
being habitually closed, the current 
passes through all relays on the line, 
the current being continuous (§ 395) 
except when a message is being sent 
from some office. When an operator, 
in sending a message, opens his key, 
the breaking of the circuit stops the 
current, demagnetizes the relays and 
allows their springs to draw back the 
armature levers, e. This breaks each 
local circuit and demagnetizes each 
sounder, the spring of which raises 
its armature. Things are now as 
shown in the diagram, which also 
represents the condition of affairs at 
every other station on the line. When 
a message is sent from any station, 
each relay lever, e, acts as a key to 




Fig. 231. 




local circuit, it and the 



322 MAGNETISM, 

sounder lever vibrating in obedience to the motions of the key at 
the sending station. Of course, the sending operator can read his 
own message from his sounder. The message may also be read from 
any sounder on the line. 

{d.) If the local circuit at New York (see Fig. 231) be lengthened 
so as to reach thence to Boston and the local battery, h, be increased 
to the dimensions of a main battery, B, (ground connections being 
made, of course), the relay at New York will transmit to Boston the 
message received from Cleveland. In such cases, the relay at New 
York becomes a repeater. Messages from New York to Chicago may 
thus be repeated at Meadville, Pa., without the intervention of any 
operator. 

446. Duplex and Quadruplex Teleg^raphy.— 

The simple Morse system, just described, is very reliable, 
but a given wire can transmit only one message at a time. 
By what is known as the duplex system, a wire may be 
made to convey two messages, one each ivay, at the same 
time, without conflict. By what is known as the quadru- 
plex syster)%, a wire may be made to carry four messages, 
two each way, at the same time, Delany's multiplex sys- 
tem enables the sending of six messages in the same direc- 
tion at one time. The student is referred to technical 
works on telegraphy for an explanation of these systems. 
A good Morse operator can send or receive thirty or forty 
words a minute ; by the aid of a combination of recent 
inventions, fifteen hundred words have been transmitted 
over a single wire in one minute. 

44T. Electric Bells.— The construction of the 
trembler or electric bell will be clearly seen by an exam- 
ination of Fig. 232. When the button at P (anywhere 
on the circuit) is pushed, two metal pieces are brought 
into contact and the circuit is thus completed. The spring 
carried by the armature of the magnet, E, makes contact 



MAGNETISM. 



323 



with the tip of the screw at C, except when it is drawn 
away by the attraction of the magnet. 




Fig. 232. 



{a.) When the spring rests against the end of the screw at (7 (the 
circuit being closed at P), the cores of E are magnetized. They 
then draw the armature away from the end of the screw and break 
the circuit at C Ey being thus demagnetized, no longer attracts its 
armature, which is thrown back against the end of the screw by the 
elasticity of the spring that supports it. It is then again attracted 
and released, thus vibrating rapidly and striking a blow upon the 
bell at ^at every vibration. (See § 459, a.) 



324 MAGNETISM. 

44:8. Making Perinaiient Magnets. — A com- 
mon way of magnetizing a steel bar is to draw one 
end of a strong magnet from one end of the bar 
to the other, repeating the operation several times, 
always in the same direction. A second method is to 
bring together the opposite poles of two magnets at the 
middle of the bar to be magnetized and simultaneously 
drawing them in opposite directions from the middle to 
the ends. A steel bar may be magnetized by striking it 
on end with a wooden mallet while it is held in the direc- 
tion assumed by the dipping needle. If a bar of steel be 
heated to redness and cooled, either slowly or suddenly, 
while lying in the magnetic meridian, it acquires magnetic 
polarity. But better than any of these can giye are the 
effects produced by electro-magnetism. 




Fig. 233. 

The bar may be permanently magnetized by drawing it, 
from its centre, in one direction over one pole of a power- 
ful electro-magnet and then, from its centre, in the oppo- 
site direction over the other pole and repeating the pro- 
cess a few times (Fig. 233). 

A bar of steel placed within a hehx through which a 



MAGNETISM. 



325 



strong current is passing will be permanently magnetized. 
The bar should be passed into one end of the helix and 
removed from the other end. 

(a,) A long, tliin, steel magnet is more powerful in proportion to 
its weight than a thicker one is. Compound magnets are, therefore, 
made of thin pieces of steel, separately magnetized and then bound 
together in bundles. A horse-shoe magnet will lift a load three or 
four times as heavy as will a bar magnet of the same weight. The 
lifting power is increased if the area of contact between the poles and 
the armature is increased. The lifting power of a magnet is strength- 
ened, in an unexplained way, by gradually increasing the load on its 
armature day by day until it bears a load which at the outset it 
could not have borne. If the load be so increased that the armature 
is torn oflP, the power of the magnet falls at once to its original 
value. The attraction between a powerful electro-magnet and its 
armature may amount to 200 lb. per square inch, or 14,000 g. per 
sq. cm. Small magnets lift a greater load in proportion to their own 
weight than large ones. A good steel horse-shoe magnet weighing 
one pound ought to lift twenty pounds* weight. 
A steel magnet loses part of its magnetism by be- 
ing jarred or knocked about and all of it by being 
heated to redness. 



449. Arixiatures. — Magnets left to them- 
selves soon lose their magnetism. They should, 
therefore, be provided with armatures. Armatures 
are pieces of soft iron placed in contact loith opposite 
poles, as shown in Fig. 234. The two poles of the 
magnet (or magnets, for two bar magnets may be 
thus protected) act inductively upon the armature 
and produce in it poles opposite in kind to those 
with which they come in contact. The poles of the 
armature in turn react upon the magnet and, by 
their power of attraction, aid in retaining the mag- 
netism. 




Fig. 234. 



450. Mag'iietic Units, — All magnetic quantities, strength of 
poles, intensity of magnetization, etc., are expressed in terms of 
special units derived from the fundamental units of length, mass and 
time, i.e., they are C. Q-. S. units. 

{a) Unit Strength of Magnetic Pole. — The unit magnetic pole is 



326 MAGNETISM, 

one of such strength that it repels a similar pole of equal strength 
with a force of one dyne when it is placed at a distance of one centi- 
meter from it. 

(6.) Magnetic Potential being measured by work done in moving 
a unit magnetic pole against the magnetic forces, the unit of mag- 
netic potential will be measured by the unit of work, the erg. 

(c.) Unit Difference of Magnetic Potential exists between two 
points when it requires the expenditure of one erg of work to 
bring a — unit magnetic pole from one point to the other against 
the magnetic forces. 

{d.) Intensity of Magnetic Field is measured by the force it exerts 
upon a unit magnetic pole ; hence, 

{e.) Unit Intensity of Field is that which acts on a unit — pole 
with a force of one dyne. 

451. Electro-Mag-iietic Units.-— The magnetic units just 
described give rise to a set of electrical units, in which the strength 
of currents, etc., are expressed in magnetic measures. (See § 320.) 

{a.) Unit Strength of Current. — A current has unit strength when 
1 cm. length of its circuit bent into an arc of 1 cm. radius (so as to 
be always 1 cm. away from the magnet-pole) exerts a force of one 
dyne on a unit maguet-pole placed at the centre. 

(5.) Unit of Quantity of Electricity is that quantity which is con- 
veyed by unit current in one second. 

(c.) Unit of Difference of Potential (or of E. M. F.) is that which 
exists between two points when it requires the expenditure of one 
erg of work to bring a unit of + electricity from one point to the 
other against the electric force. 

{d) Unit of Resistance. — A conductor possesses unit resistance 
when unit difference of potential between its ends causes a current 
of unit strength to flow through it. 



4:52. Practical Units.— As some of these " abso- 
lute ^^ electro-magnetic units are too large for common, 
convenient use and others are too small, the practical 
units, the volt, the ohm, the ampere and the coulomb have 
been chosen and are generally used. These units have 
been already described, the value of each in absolute elec- 
tro-magnetic units being given. 



MAGNETISM. 327 

453. Molecular Changes in a Magnet. — 

When a steel or iron bar is strongly magnetized, it in- 
creases in length and diminishes in thickness. This effect 
is probably due to the magnetization of the individual mole- 
cules, which tend to set themselves parallel to the length 
of the bar. This supposition is confirmed by the observa- 
tion that at the moment when a bar is magnetized or 
demagnetized, a faint metallic click is heard in the bar. 
When a tube containing water rendered muddy with finely 
divided magnetic oxide of iron is magnetized, the liquid 
becomes clearer in the direction of magnetization, the par- 
ticles apparently setting themselves end to end and al- 
lowing more light to pass between them. A piece of iron, 
when powerfully magnetized and demagnetized in rapid 
succession, grows hot, as if the changes were accompanied 
by internal friction. 

454. Theory of Magnetism. — These and other 
phenomena point to a theory of magnetism very different 
from the old notion of '' magnetic fluids." It appears that 
every molecule of a magnet is itself a magnet and that the 
molar magnet becomes a magnet only by the molecular 
magnets being turned so as to point one way. This con- 
clusion is supported by the observation that if a glass tube 
full of iron filings be magnetized, the filings may be seen 
to set themselves endwise and that, when thus once set, 
they act as a magnet until they are shaTcen up. 

455. Relation of Magnetism to Energy. — A 

magnet is a reservoir of potential energy. This energy is 
due to the expenditure, at some time, of a definite amount 
of energy, of some kind. By virtue of its potential energy, 



328 MAGNETISM. 

it can do a definite amount of work and no more. For in- 
stance, it may attract a certain amount of iron. When 
thus fully loaded, the magnet has done its full work and 
can do no more. When the iron is torn from the 
magnet, more energy is expended and the magnet 
thus endowed again with potential energy, A 
magnet has not an inexhaustible supply of energy, 
as some have supposed. 

EXEKCISES. 

1. (a.) What is a magnetic pole? (6.) A magnetic equator? 
(c.) How does a magnet behave toward soft iron? (cZ.) How does 
soft iron behave toward a magnet ? 

2. (a.) State carefully the various effects that one magnet may 
exert upon a second magnet. (&.) Generalize these observed facts 
into a law. 

3. On board an iron ship that is laying a submarine telegraph 
cable, there is a galvanometer used for testing the continuity of the 
cable. It is necessary to prevent the magnetized needle of the gal- 
vanometer from being affected by the magnetism of the ship. How 
can this be done ? 

4 ((^.) Given a bar magnet, how would you determine the sign of 
either of its poles? (5.) What is a diamagnetic substance ? 

5. If a magnetic needle be freely suspended from its centre of 
gravity, what position will it assume ? 

6. (a.) Do you think that the earth is a magnet? (&.) Give a good 
reason for your answer, (c.) Do the magnetic and the geographical 
meridians ever coincide ? (d) Do they always coincide ? (^.) If 
they do not coincide, what name would you give to their difference 
in direction ? 

7. («.) Does the magnetic attraction of the earth upon a ship's 
compass tend to float the ship northward ? (&.) If so, why? If not, 
why not ? 

8. (a.) State and illustrate the second law of motion. (6.) State 
and illustrate the law of universal gravitation, (c.) A body falls to 
the ground from rest in 11 seconds ; what is the space passed over? 

9. An electric bell in Cleveland, Ohio, is to be rung by a battery 
in New York City. Should the magnet coils of the bell be made of 
fine or coarse wire ? 



MAGNETISM, 



329 



10. Would you use a long coil or a sliort coil galvanometer to 
measure the current used to ring the bell above mentioned ? 

11. Would it make any difference whether the galvanometer were 
put into the circuit at New York or at Cleveland if the line be thor- 
oughly insulated ? 

12. With a local battery of 2 cells, each having an internal resist- 
ance of 2 ohms, what should be the resistance of the sounder? 

13. The cells represented in Fig. 235 have each an E. M. F. of 2 
volts and an internal resistance of 3 ohms. 

What is the resistance of the external cir- 
cuit, (7, if the battery is arranged in the best 
possible way ? Ans, 2 ohms. 

14. Why is it that when there is little other 
resistance in the circuit, a stout wire with few 
turns will make a stronger electro-magnet than 
a very fine wire with many more turns? 

15. A battery of 5 Leclanche cells was con- 
nected in simple circuit with a galvanometer 
and a box of resistance coils. A deflection of 
40" having been obtained by adjustment of the 
resistances, it was found that the introduction 
of 150 additional ohms of resistance brought 
down the deflection to 29°. A battery of ten 
Daniell's cells was then substituted in the cir- 
cuit and adjusted until the resistance was 40° 
as before. But this time it was found that 216 

ohms had to be added before the deflection was brought down to 
29°. Taking the E. M. F. of a single Daniell's cell as 1.079 volt, 
calculate that of a single Leclanche cell. Ans. 1.499 volt. 

16. An electric bell has a resistance of 0.5 ohm. It requires a 
current of 20 milliamperes to ring it. It is on a line of 1 mile 
of No. 20 copper wire (see Appendix I). Ignoring the internal re- 
sistance of the battery, find how many Leclanche cells (E. M. F. 
=r 1.5 volts) will be required. 

17. We have to send a current through a telegraph line, 100 miles 
long, the resistance of which is 13 ohms per mile. The battery is 
composed of Daniell cells, each having an E. M. F. of 1.079 volts and 
an internal resistance of 2 ohms. The telegraphic instrument offers 
a resistance of 130 ohms and requires a current of 10 milliamperes 
to w^ork it. Will one cell of battery answer our purpose? Why ? 

18. Under what circumstances will a magnet repel an unmag- 
netized piece of iron ? 




Fig. 235. 



330 MAGNETISM. 

19. Give two or three differences between electric attractions and 
repulsions and magnetic attractions and repulsions. 

20. A zinc and a copper plate are respectively united by copper 
wires to the terminals of a galvanometer. They are dipped, side by 
side, into a glass containing dilute sulphuric acid. The galva- 
nometer needle, at first, shows a deflection of 28°, but five minutes 
later the deflection has fallen to 11°. How do you account for this 
falling off ? 

21. A wire, the resistance of which was to be determined, was 
placed in a Wheatstone's bridge, in which resistances of 10 and 100 
ohms respectively were used as the fixed resistances. Its resistance 
was balanced when the adjustable coils were arranged to throw 281 
ohms into circuit. What was its resistance ? (See Appendix M, [e.'].) 

Ans. 25.1 ohms. 

22. Relays are wound with long, fine wire and sounders with 
short, stout wire. Why is there this difference ? 



MAGNETISM. 331 

Recapitulation.— To be amplified by the pupil for 



review. 



r NATURAL. 



ARTIFICIAL. 



r MAGNETS. 



Permanent. . 



MOLECULAR. 



Temporary or 
Electro-Magnets. 



Magnets. 
Changes. 



Forms. 
How Made, 

' Definition, 

A dvantages. 

Forms. 

Residual 
Magnetism. 

Telegraph. 

Electric Bells. 



POLES. 

CHARACTERISTICS. 

LAWS. 

Magnetic 

•iamagnetic. 



RELATION TO 



( M/ 
\ Dl 



>• Substances. 



l-H 

Si 

o 
< 



RETENTIVITY. 
THEORY. 

MAGNETIZATION. 



BY CONTACT. 

MODES OF. 

BY INDUCTION. - 



f Magnetic Field, 
Lines of Force. 
Precedes Attraction. 
Magnetic Screens. 



TERRESTRIAL \ 



f POLES. r Compass. 

MAGNETIC NEEDLES. \ Dipping. 
DIP. \ Astatic. 

DECLINATION. 



MAGNETIC AND ELECTRO-MAGNETIC UNITS. 
L RELATION TO ENERGY. 



J ^^EC TtON V. 

INDUCED ELECTRICITY. 

456. Induced Cvirrents.^ — From our study of 
frictional electricity and magnetism^ we are familiar with 
the term induction^ by which we understand the influence 
that an electrified body exerts upon a neighboring unelec- 
trified body or that a magnetized body exerts upon a 
neighboring magnetic but unmagnetized body. In 1831, 
Faraday discovered an analogous class of phenomena 
which we are now about to consider. An induced cur- 
rent is a current produced in a conductor by the 
influence of a neighboring current or magnet, A 
current used to produce such an efiect is called an in- 
ducing current. 

45T. Inductive Effect of Closing or Break- 
ing a Circuit. — In Fig. 236, B represents a double 
coil made as follows: On a hollow cylinder of wood or 
card-board are wound several layers of stout, insulated, 
copper wire. The two ends of this wire, which constitutes 
the primary coil, are seen dipping into the cups, gg'. Upon 
this coil and carefully insulated from it, is wound a much 
greater length of finer, insulated copper wire. The two ends 
of this wire, which constitutes the secondary coil, are seen 
connecting with a delicate, long coil galvanometer, G, 



INDUCED ELECTRICITY, 



333 



Remember that there is no electrical connection between 
the two coils. Wires from the poles of a voltaic cell, P, 
dip into mercury in the cups g g', thus closing a circuit 
through the primary coil of B. While this circuit is closed, 
the galvanometer needle is at rest, showing that no 
current is passing through the secondary coil. By lifting 
one of the wires from its cup, the inducing current is 
interrupted. At this instant, the galvanometer needle 




Fig. 236. 



is deflected, as by a sudden impulse that immediately 
passes away. This movement of the galvanometer needle 
shows the existence of a momentary, induced current in 
the secondary coil. The direction in which the needle 
turns, shows that the secondary current is direct, i, 6., 
that it has the same direction as the inducing cur- 
rent. If the wire just removed from the cup be replaced 
and the inducing current thus re-established, the galva- 
nometer needle will be momentarily turned in the direction 
opposite to that in which it was previously turned. W%eri 
a current begins to flow through the primary coil, 
it induces a current in the secondary coil. When 



334 INDUCED ELECTRICITY. 

it ceases to fioiv through^ the primary coil, a cur- 
rent flowing in the opposite direction is induced 
in the secondary coil. Both induced currents are 
merely momentary in duration, 

458. The Extra Current. — When a circuit is 
made or broken, each convolution of a coil placed in the 
circuit acts inductiyely upon the other convolutions of 
the coil as if they were portions of two unconnected cir- 
cuits. This action is called the induction of a 
current upon itself ; the current thus produced is 
called the extra current. 

(a.) When tlie circuit is made, the extra current is inverse or 
opposite in direction to the primary current and acts against it. 
The extra current at the breaking of the circuit is direct and adds 
its effect to that of the primary current. Hence, a spark is more 
often seen on breaking than on making contact. Increasing the 
number of coils or convolutions in the circuit will increase the brill- 
iancy of the spark. If the coil has an iron core (electro-magnet) 
the effect is especially marked. 

459. Ruhmkorff's Coil. — The induction coil, 
often called, from the name of its inventor, Ruhmkorff's 
coil, is a contrivance for producing induced cur- 
rents in a secondarij coil hy closing and opening, 
in rapid succession, the circuit of a current in 
the primary coil. The essential parts are described in 
§ 457. In the complete instrument, the axis of the coils 
is a bundle of soft iron wires. These wires usually ter- 
minate in two small plates of soft iron which thus form 
the ends of the wire bundle. Around this bundle, is wound 
the primary coil of stout, insulated, copper wire. Upon 
the primary coil, but carefully insulated from it, is wound 



INDUCED ELECTMiriTY. 



335 




Fig. 237. 



the secondary coil which is made of a great many turns of 
fine, silk covered, copper wire. 

{a.) The wire bundle {M, Fig 238) becomes magnetized by the 
action of the battery current in the primary coil and then adds its 
inductive eflPect 
upon the secondary 
coil to the effect of 
the primary itself. 
The primary cir- 
cuit is rapidly 
broken and closed 
by an automatic in- 
terrupter or contact 
breaker, repre- 
sented at the left 
hand of the coil, 
Fig. 237, and at the 
right hand of the 

diagram in Fig. 238. One of the posts there seen carries an elastic, 
metallic, vibrating plate with an iron hammer, h, at its end. This 
hammer vibrates back and fortli between the end of the iron core of 
the coils and the end of the metal adjusting screw, d, which is car- 
ried by the other post seen in the figure. These posts are in the 

primary circuit. When 
the hammer rests against 
the end of the adjusting 
screw, the circuit is closed 
and the iron core is mag- 
netized. As soon as the 
core is magnetized, it at- 
tracts the hammer, thus 
drawing it away from the 
end of the screw and break- 
ing the circuit. As soon as the circuit is broken, the bar is de- 
magnetized and the plate, by virtue of its elasticity, throws the 
hammer back against the screw, closing the circuit and again mag- 
netizing the core. The plate is thus made to vibrate with great 
rapidity, each oscillation making or breaking the primary circuit and 
creating a series of induced currents in the secondary coil. 

(b.) The condenser {C G, Fig. 238), which is generally placed in 
the pedestal or base of the coil, consists of a number of sheets of 
tinfoil insulated from each other by thin sheets of varnished paper 




336 INDUCED ELECTRICITY, 

or oiled silk. Alternate layers of the tinfoil are connected, i. e,, the 
first, third, fifth, seventh, etc., layers are connected, as also are the 
second, fourth, sixth, eighth, eU., thus forming two separate, in- 
sulated series. One series {e. g., the odd numbered sheets) is con- 
nected with one of the posts of the contact breaker ; the other series, 
with the other post. Thus, the plates of the condenser do not 
form a part of the primary circuit but are, as it were, lateral expan- 
sions of that circuit, one on each side of the contact breaker. The 
effect of the condenser is to lessen the spark when the primary cir- 
cuit is made or broken and to increase the force of the discharge of 
the secondary coil. 

(c.) For an ordinary Ruhmkorff's coil, one to three Bunsen or 
potassim di-chromate elements will suffice. 

{d.) Most induction coils are provided with a commutator, for the 
purpose of changing the direction of the current through the primary 
coil and, consequently, the direction of the currents induced in the 
secondary coil. One form of the commutator is shown at the right 
hand end of Fig. 237. It is not an essential part of the instrument. 

Experiment 104. — Let the members of the class join bare hands. 
Let the pupil at one end of the line place a finger on one of the 
binding posts or electrodes of the secondary coil of a small induction 
coil. Then let the pupil at the other end of the line, momentarily 
touch the other electrode. Each person in the line will feel a 
** shock. '* The experiment should not be tried with a powerful coil, 
as the spasmodic, muscular contractions thus produced are sometimes 
painful and permanently injurious. 

460. Spark from the Induction Coil. — If the 

ends of the secondary coil be connected, opposite currents 
alternately traverse the connecting wire. When the ends 
are disconnected, the inverse current cannot overcome the 
resistance of the intervening air because of its low electro- 
motive power (§ 458, a). The direct current, produced 
hy breaking the primary circuit, is alone able to 
force its way in the fbrm of a sparh. The sparks 
vary with the power of the instrument. 

(a.) Mr. Spottiswoode, of London, has made an induction coil, the 
secondary coil of which contains 280 miles of wire wound in 340,000 
turns. This magnificent instrument has a resistance of more than 



INDUCED ELECTRICITY, 



337 



100,000 ohms, and, when worked with a battery of 30 Grove cells, 
yields a spark 42J inches long, — a result gre::ter than that obtainable 
from any electric machine. The induction coil may be used to pro- 
duce any of the effects of frictional electricity, it being at the same 
time nearly free from the limitations that atmospheric moisture 
places upon ordinary electric machines. 

(5.) For many instructive and beautiful experiments with this in- 
strument and other information relating thereto, see the little book, 
'' Induction coils : — How made and how used," published by D. Van 
Nostrand, New York ; Price, 50 cents. 

461. Currents Induced by Change of Dis- 
tance. — If the primary coil be made movable, as shown 
in Fig. 239, and, with a current passing through it, be 
suddenly placed within the sec- 
ondary coil, the galvanometer 
will show that an inverse current 
is induced in the outer coil. 
When the needle has come to 
rest, let the primary coil be re- 
moved and the galvanometer 
will show that a direct current 
is induced. Prom this we see 
that when the primary coil, 
hearing a current, is brought 
near or thrust into the sec- 
ondary coil, an inverse cur- 
rent is induced in the latter ; 
that when the coils are sep- 
arated, a direct current is 

induced in the secondary coil; that the induced 
currents flow ivhile a change of distance is varying 
the inductive effect of the primary current, Re- 
mo\nng the primary coil to an infinite distance is equivar 
lent to breaking its circuit, as in § 457. 




Fig. 239. 



338 



INDtJCEi) ELECTRICITT, 



463. Magneto -Electric Currents. — We have 
already noticed that there is an intimate relation between 
electric and magnetic action. We have seen that an elec- 
tric current may develop magnetism. Faraday found that 
electricity may be developed by magnets ; the results of this 
discovery have already become of incalculable commercial 
importance. If, instead of the primary coil bearing the 




Fig. 240. 



inducing current^ a bar magnet be used, as shown in Fig. 
240, the effects produced will be like those stated in the 
last paragraph. When the magnet is thrust into the 
interior of the coil, an induced current ivill floiv 
while the motion of the magnet continues. When 
the magnet becov^es stationary, the current ceases to 
flow and the needle of the galvanometer gradually 
comes to rest. When the magnet is withdrawn, an 
induced current floius in the opposite direction. 
Of course, it makes no difference whether the magnet be 



INDUCED Electricity, 



B39 



moved toward the coil or the coil be moved toward the 
magnet. The more rapid the motion, the greater will be 
the electromotive force of the induced currents. 

463. The Inductive Action of a Temporary 
Magnet.— If within the coil, a soft iron bar (or still 
better, a bundle of straight, soft, iron wires) be placed, 
as shown in Fig. 241, the induced current may be more 




Fig. 241. 

effectively produced by bringing one end of a permanent 
magnet near the end of the soft iron. In this case, the 
induced currents are due to the varying magnetism of the 
soft iron, this magnetism being due, in turn, to the in- 
ductive influence of the permanent magnet. Thus w^e 
see that ivhen the intensity of the magnetism of a 
har of iron or steel is increased or diminished, 
currents are induced in the neighboring coil. 
Similar effects may be produced by moving one pole of 
the magnet across the face of the coil from end to end. 



B40 



INDTTCED ELECTRtCXTF, 




464. The Wheel Armature.— Imagine the soft 
iron bar in the helix of Fig. 341 to be grooved and several 
times as long as the helix through which it passes. Imag- 
ine the ends of this bar to be brought together so as to 

form a complete iron 
ring carrying oue helix. 
If the number of helices 
upon the ring be in- 
creased to twelve we 
shall have the wheel 
armature^ shown^ in an 
unfinished condition, in 
Fig. 242. If the pole 
of a magnet be passed 
around the face of this 
wheel, it will pass twelve 
coils of wire and induce 
a current of electricity as it approaches each coil and an 
opposite current as it leaves each coil^ thus inducing 
twenty-four currents for each revolution. Of course^ it' 
makes no difference whether the magnet be permanent or 
temporary, whether the pole of the magnet moves iy the 
coil or the coil passes iy the pole of the magnet. Then, if 
the magnet be fixed and the wheel turn upon its axis in 
such a way as to carry its coils across the end of the 
magnet, we shall be inducing twenty-four currents of 
electricity for each revolution of the wheel. This is what 
happens in the operation of a dynamo-electric machine. 
Whe1^ a closed circuit conductor moves in a mag- 
netic field so as to cut across the lines of magnetic 
force (§ 433), an induced current of electricity flows 
through the conductor in one direction while the 



Fig. 242. 



INDUCED ELECTRICITY, 



341 



conductor is approaching the point of greatest 
magnetic intensity and in the opposite direction 
while the conductor is moving away from such 
point of maximum intensity. The varying magnetic 
intensity of the iron core of each moving coil increases 
this effect as explained in § 463. Of course, the number 
of coils on the armature may be more or less than twelve, 
or the armature may be of a form almost wholly different 
from that just described, but, in every case, the principle 
of its action is as above stated. The dynamo represented 
in Fig. 243 has only eight armature helices and diametric- 
ally opposite coils are joined so as to form four pairs. 




Fig. 243. 

465. Dynamo-Electric Machines. — In the 

Brush dynamo-electric machine, represented in Fig. 243, a 
shaft runs through the machine from end to end, carrying 
a pulley, P, at one end, a commutator, c, at the other, 
and a wheel armature, R, at the middle. The armature, 
R, carries eight or more helices of insulated wire, H H, 



343 INDUCED ELECfitlCITT, 

As the shaft is turned by the belt acting upon P, R and 
c are turned with it. As R turns around, it carries the 
eight coils^ H H, rapidly across the poles of the four 
powerful field magnets, M M, 

As each coil passes each pole, it necessarily tra- 
veT'ses the magnetic field and cuts across the lines 
of magnetic force; consequently, currents are in- 
duced in the coil. These currents are carried on insu- 
lated wires to the commutator rings, c c, where they are 
united in such a way as all to flow in the same direction, 
forming a continuous current. The electricity is taken 
from the revolving commutator, c c, by the four or more 
fixed, copper plates, i i, technically called " brushes," then 
carried down the flexible copper strips, s s, then passed 
through the insulated wire of the electro-magnets, M if, 
and, finally, to the + binding post. Thence the current 
passes by a wire to the external circuit, e. g,, to an arc 
lamp (Fig. 246) and from this to a second lamp, and so on 
through all of the lamps of the circuit and from the last 
lamp back to the — binding post of the dynamo-electric 
machine, thus making the circuit complete. Sixty or 
more arc lamps in series may be worked by one of these 
machines. No part of the circuit of a dynamo should 
have an earth connection. The complete circuit (except 
through the lamp carbons) should be of carefully insu- 
lated wire. 

Dynamo-electric machines are being rapidly introduced 
for purposes of electric lighting, electro-plating, motive 
power, telegraphy, etc. They are made in various forms, 
but the principle underlying the action of them all is the 
same as that stated in the last paragraph. After master- 
ing the action of one dynamo-electric machine the pupil 



INDVCEB ELECTRICITY. 343 

will have little trouble in understanding the action of any 
other that he may have a chance to examine. Dynamo- 
electric machines are often called " dynamos.'^ A small, 
hand power dynamo, suitable for school use, may be had 
for $30 or more. 

{a.) In cases where a higli E. M. F. is needed (as in arc electric 
lighting), the armature helices are wound with many turns of wire 
which gives a high internal resistance. Compare § 399. When a 
smaller E. M. F. is wanted (as in direct, incandescence electric light- 
ing or in electro-plating), fewer turns of wire of greater diameter 
are used. This reduces the internal resistance of the dynamo. 
Compare § 400. The E. M. F. will vary with the strength of the 
magnetic field and the speed at which the armature is revolved. 
Thus, a given dynamo may be run slowly for a few lamps and at a 
higher speed for a greater number of lamps. In practice, however, 
special automatic devices are generally provided for adapting the 
E. M. F. to the varying resistances of the external circuit without 
changing the speed of the dynamo. 

(6.) If permanent magnets are used instead of electro-magnets, 
the machine is called a magneto -electric instead of a dynamo-electric 
machine. Small magnetos (armatures wound with long, thin wires) 
are much used for electro-medical purposes. The patient holds two 
metallic handles connected with the terminals of the instrument 
and receives a rapid succession of shocks when the armature is 
turned. 

{c.) If, instead of expending mechanical energy to turn the shaft 
of the dynamo and thus produce an electric current, we pass a strong 
current of electricity through the dynamo, the shaft of the dynamo 
will be turned in the opposite direction and may be made to drive 
ordinary machinery as an electric motor. In the former case, we 
convert mechanical energy into electric energy ; in the latter case, 
we convert electric energy into mechanical energy (§ 473). 



466. Tncandesceiice Electric Lamps.— When 
a conductor of high resistance is heated to incandescence 
by the passage of a current, we have an illustration of the 
fundamental principle of incandescence electric Hght- 
ing. To prevent the fusion of the conductor, a carbon 



344 



INDUCED ELECTRICITY, 



THE SWAN ELECTRIC LAMP. 



filament^ about the size of a horse-hair, is used — carbon 
never having been melted. To prevent the combustion of 

the carbon filament, it is enclosed 
in a glass globe containing either 
a high vacuum or only some inert 
gas, incapable of acting chemic- 
ally upon the carbon at even the 
high temperature to which it is 
to be subjected. The ends of the 
carbon are connected with plat- 
inum wires that are fused into 
and passed through the glass. 




(a.) The filament is carbonized in 

different ways and given different 

shapes by different inventors. The 

Edison carbon is made of bamboo fibre 

and is in the shape of an ordinary hair 

pin. The Swan carbon is made of 

parchmentized cotton thread. Fig. 

244 represents the Swan incandescence 

lamp and is half the actual size of the 

standard sixteen candle power lamp. 

Incandescence lamps are generally 

Fig. 244. operated abreast, as shown in Fig. 245, 

being placed, as it were, in little 

bridges of wire connecting the two conductor '* mains." Thus, the 

resistance of the circuit is reduced hy the successive addition of lamps. 

(b.) The resistance of carbon is 
lowered by heating the conductor. 
The " hot " resistance of an incan- 
descence lamp is about f its " cold " 
resistance. 

Fig. 245. 

467. The Voltaic Arc— 

The most brilhant luminous effect of current electricity is 
the arc of an electric lamp. This lamp consists essentially 
of two pointed bars of hard carbon, generally copper coated 



'^um 



INDUCED ELECTRICITY. 



345 



(Experiment 78), placed end to end in the circuit of a 
powerful current. If the ends of the carbons be separated 
a short distance while 

the current is passing, ^^ ^^^^° electric lamp. 

the carbon points be- 
come intensely heated 
and the current will 
not be interrupted 
thereby. When the 
earbons are thus 
separated, their tips 
0low with a brill- 
iancy ivhich ex- 
ceeds that of any 
other light under 
h u m a n control, 
while the tempera- 
ture of the inter- 
vening arc is un- 
equalled by any 
other source of ar- 
tificial heat. 

The mechanism 
shown in the upper 
part of Fig. 246, is for 
the purpose of auto- 
matically separating the carbons and ^^ feeding'^ them 
together as they are burned away at their tips and for the 
purpose of cutting the lamp out of the circuit in case of 
any irregularity or accident. Such lamps of from one to 
two thousand candle power and requiring an expenditure, 
at the dynamo, of about one-horse power jjer lamp are 




Fig. 246. 



346 



INDUCED ELECTRICITY, 



now quite common. Lamps of a hundred thousand can- 
dle power have been made. The current may be furnished 
by a battery of forty or more Groye^s cells but, for eco- 
nomical reasons, it is almost uniyersally supplied by a dy- 
namo-electric machine. 

{a.) It is necessary to bring the carbons into contact to start tbe 
light. The tips of the carbons become intensely heated on account 
of their small area of contact and the consequent high resistance at 
that point. The carbon (and its usual copper coating) begins to 
volatilize. When the carbons are separated, the current is kept up 
by this intervening layer of vapor and the accompanying disin- 
tegrated matter, which act as a conductor. Arc lamps are generally 

operated in series, so that the 



current passes in succession 
through all the lamps on the 
circuit. TJie resistance of the 
circuit is thus increased hy the 
successive addition of lamps, 

iff,) The constitution of the 
voltaic arc may be studied by 
projecting its image on a screen 
with a lens. Three parts will 
be noticed : 

1. The dazzling white, con- 
cave extremity of the 
positive carbon. 

2. The less brilliant and more 
pointed tip of the nega- 
tive carbon. 

3. The globe shaped and beau- 
tifully colored aureole 
surrounding the whole. 

(c.) There is a transfer of mat- 
ter across the arc in the direc- 
tion of the current, the positive 
carbon wasting away more than 
twice as rapidly as the negative. 
Most of the light of the lamp is 
radiated from the crater at the end of the positive carbon. If the 
arc be too short, many of these rays will be intercepted by the nega- 
tive (generally the lower) carbon, thus lessening the efficiency of the 




Fig. 247. 




INDUCED ELECTRICITY, 347 

lamp. If the arc become too long, it will ** flame" and much of 
the light thus be lost. If the electrodes be horizontal, the arc will 
be curved upward by ascending air currents. Arc lamps are now 
largely used for lighting streets, factories, stores, etc., many thou- 
sands having been sold in every quarter of the globe (§ 000). 

468. The Telephonic Current. — An electric cur- 
rent may be induced in a coil of insulated wire surround- 
ing a bar magnet by the ^ 
approach and withdrawal 
of a disc of soft iron. The 
disc, a (Fig. 248), is mag- 
netized by the inductive Fig~^ 8 "* 
influence of the magnet, m, 

(§ 435). The disc, thus magnetized, reacts upon the 
magnet, m, and changes the distribution of magnetism 
therein. By varying the distance between a and m, the 
successive changes in the distribution of the magnetism of 
m induce to-and-fro currents in the surrounding coil 
(§ 463). When a approaches m, a current flows in one 
direction ; when it recedes, the current flows in the oppo- 
site direction. 

469. The Telephonic Circuit. — If the wire sur- 
rounding the magnet mentioned in the last paragraph be 
continued to a distance and then wound around a second 
bar magnet, as shown in Fig. 249, the currents induced at 
M would affect the magnetism of the bar at M' or the in- 
tensity of its attraction for the neighboring disc, a'. A 
vibratory motion in the disc, «, would induce electric 
currents at M\ these currents, when transmitted to M', 
perhaps several miles distant, would affect the magnetism 
of the bar there and tend to produce exactly similar vibra- 



348 



INDUCED ELECTRICITY, 



tions in a'. " It is as if the close approach and quick 
oscillation of the piece of soft iron fretted or tantalized 



LIN£ WIRE 




Fig. 249. 

the magnet and sent a series of electrical shudders through 
the iron nerye/^ When the current generated at M flows 
in such a direction as to reinforce the magnet at M\ the 
latter attracts a^ more strongly than it did before. When 
the current flows in the opposite direction, it weakens the 
magnetism of M\ which then attracts a! less. The disc, 
therefore, flies back. Thus, the vibrations of a! are like 
those of a, 

{a.) We have here the principle of the telephone, so far as electric 
action is involved. Further consideration of this instrument must 
be deferred until we have learned more concerning sound. (See 
§ 504.) 




INDUCED ELECTRICITY. 349 



Exercises. 

1. A dynamo is feeding 16 arc lamps, the average resistance of 
each of which is 4.56 ohms. The internal resistance of the dynamo 
(^.e., of the wire conductors of the armature and field magnets) is 
10.55 ohms. What current does the dynamo yield? 

Ans. 10.04 amperes. 

2. If a wire about 18 inches long be 
attached to one electrode of a potassium 
dichromate cell and the other electrode 
momentarily touched with the other end 
of the wire, a minute spark may be 
noticed at the instant of breaking the cir- Yig, 250. 
cuit. If the wire be bent into a scalari- 

form or ladder like shape and the experiment repeated, the spark will 
be greater than before. If the form of the external circuit be again 
changed by winding the wire into a spiral (as shown in Fig. 250), the 
spark will be still greater. Explain the repeated increase in the spark. 

3. A dynamo is run at 450 revolutions, developing a current of 
9.925 amperes. This current deflects the needle of a tangent gal- 
vanometer, 60°. (See Appendix L.) When the speed of the dynamo 
is sufficiently increased, the galvanometer shows a deflection of 74°. 
What is the current developed at the higher speed ? 

Ans. 20 amperes. 

4. The current running through the carbon filament of an incan- 
descence lamp was found to be 1 ampere. The difference of poten- 
tial between the two terminals of the lamp was found to be 30 volts. 
What was the resistance of the lamp ? 

5. A yard of silver wire weighs 7.2 grains and has a resistance of 
0.3 ohm. What is the resistance of a foot of silver wire that weighs 
one grain? Ans. 0.24 ohm. 

6. If a pure copper wire has a weight of one grain and a resistance 
of 0.2106 ohms per foot and a commercial copper wire has a weight 
of 164 grains and a resistance of 0.547 ohms per 20 ft., what is the 
percentage conductivity of the latter as compared with pure copper? 

Ans. 93.9 per cent. 

7. I want to place, in series, 10 incandescence lamps, each of 25 
ohms resistance ; the line wire is to be 200 feet long and must have 
not more than 2 per cent, of the resistance of the lamps. Determine 
from the table in Appendix I what size of mre (American gauge) 
should be used, Ans. No. 24. 



350 INDUCED ELECTRICITY, 

8. I want to place the same lamps abreast. The line wire is to be 
200 feet long and have a resistance of not more than 2 per cent, that 
of the lamps. Determine from the table what size wire should bv3 
used. Ans. No. 4 (B. & S.) 

9. What length of No. 0000 pure copper wire (B. & S.) will have a 
resistance of 1 ohm ? (See Appendix I.) Ans. 19607.84 ft. 

10. A dynamo has an E. M. F. of 206 volts and an internal (or in- 
terpolar) resistance of 1.6 ohms. Find the current strength when 
the external resistance is 25.4 ohms. Ans. 7.6 amperes. 

11. A dynamo has an internal resistance of 2.8 ohms. The line 
wire has a resistance of 1.1 ohms and joins the dynamo to 3 arc lamps 
in series, each lamp having a resistance 3.12 ohms. Under such 
conditions, the dyoamo develops a current of 14.8 amperes. What 
is the E. M. F. ? Ans, 196.25 volts. 

12. A dynamo, run at a certain speed, gives an E. M. F. of 200 
volts. It has an internal resistance of 0.5 ohm. In the external 
circuit are 3 arc lamps in series, each having a resistance of 2.5 ohms. 
The line wire has a resistance of 0.5 ohm. I want a current of just 
25 amperes. Must I increase or lessen the speed of dynamo? 

13. With an external resistance of 1.14 ohms, a dynamo develops a 
current of 81.58 volts and 29.67 amperes. What is the internal re- 
sistance of the dynamo ? Ans. 1.61 ohms. 

14. Upon trial, it was found that a dynamo that was known to 
have an internal resistance of 4.58 ohms developed a current of 157.5 
volts and 17.5 amperes. What was the resistance of the external 
circuit? Ans. 4.42 ohms. 

15. Three incandescence lamps having a resistance of 39.3 ohms 
each (when hot) were placed in series. The total resistance of the 
circuit outside of the lamps was 11.2 ohms. The current measured 
1.2 amperes. What was the E. M. F. ? Ans. 154.92 volts. 

16. The same lamps were placed in multiple arc with another 
dynamo. The line wire was adjusted so that its resistance with the 
internal resistance of the machine was 11.2 ohms as before. The 
current was 1.2 amperes. What was the E. M. F.? 

Ans. 29.16 volts. 

17. A dynamo supplies current for two incandescence lamps in 
series, each having a hot resistance of 97 ohms. The other resist- 
ances of the circuit amounted to 12 ohms. The current in the first 
lamp was 1 ampere. What was the current carried by the carbon 
filament of the second lamp ? What was the E. M. F. ? 

18. The resistance of the normal arc of an electric lamp is 3.8 
ohms. The current strength is 10 amperes. What is the difference 
of potential between the carbon tips. Ans. 38 volts. 



INDUCED ELECTRICITY. 351 

19. The resistance of the arc lamp above mentioned, when the 
carbons are held together, is 0.62 ohm. When it is burning with 
normal arc and a 10 ampere current, what is the dijfiference of poten- 
tial between the terminals of the lamp ? Ans. 44.2 volts. 

HONOKART PROBLEMS. 

20. Four arc lamps, with a resistance of 6 ohms each, are joined 
in series, 150 feet apart. The first lamp is 1,500 feet and the last 
is 1,850 feet from the dynamo. The line wire has a conductivity 
of 96 per cent, that of pure copper. Its resistance must not exceed 
8 per cent, of that of the lamps. The resistance of a foot of pure 
copper wire 1 mil in diameter being 9.94 ohms, what must be the 
diameter of the line wire ? Ans. 133 mils or 0.133 inch. 

Use No. 10 wire, B. W. G. (A pp. I). 

21. Twenty-five similar voltaic cells having an internal resistance 
of 15 ohms each were joined in series, by short and stout copper 
wires to a 70 ohms incandescence lamp and produced a current of 
0.112 ampere. What would be the strength of the current sent by 
a series of 30 such cells through a series of 2 lamps, each of 30 ohms 
resistance? Ans. 0.118 ampere. 

22. What would have been the strength of current through the 
two lamps if the area of each of the battery plates had been doubled, 
all things else remaining the same ? Ans. 0.215 ampere. 

28. I join 50 arc lamps in series. Each lamp has a resistance of 
4.5 ohms. The line wire connecting them with the dynamo is 34 
miles long and its conductivity is 90 per cent, that of pure copper. 
One tenth of the total energy of the external circuit is lost in heat- 
ing this line wire. What is its diameter, it being assumed that 1 foot 
of pure copper wire. 1 mil in diameter has a resistance of 9.94 ohms. 

Ans. 90.8 mils. 
Use No. 11 wire (B. & S.) 



353 



INDUCED ELECTRICITY. 



Recapitulation, — To be amplified by the pupil for 



review 


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- MAGNETS. 



PRIMARY CIRCUIT. - 



Primary. 

Secondary. \ Coils. 
ruhmkorff. 
Extra Current. 

CHANGE OF DISTANCE OF PRIMARY CIRCUIT. 

{Current. 
Circuit. 
r PERMANENT. 

*- Magneto-Electric Machines. 
[ Wheel Armature. 

f Electro-Plating. 



Incandescence 
Electric Light' 
ing. 
Dynamo-Elec- 
tric Machines, \ Arc Electric 



^TEMPORARY. J 



Used for.... 



Lighting, 

Charging Stor^ 
age Batteries. 

Motive Power, 



^ Electric Motors. 



)SECTiON VL 



ELECTRIC CURRENTS AS RELATED TO HEAT 
AND MECHANICAL WORK. 

470. The Convertibility of Electric En- 
ergy. — Whenever an electric current does work of any 
kind, it does it at the expense of a part of its own energy. 
Anything that increases the resistance of a circuit, decreases 
the strength of the current (§ 386). But such a diminu- 
tion may be caused by a counter electromotive force set up 
somewhere in the circuit. The E. M. F. of polarization 
is an example of the truth under consideration. When- 
ever a current is used to drive an electric motor, the action 
of the motor generates a back current that diminishes the 
current of the battery or dynamo. All of the current 
that is not eocpendecl in some such way^ in exter- 
nal ivorh, is dissipated as heat. The dissipation may 
be in the battery (or dynamo), in the external circuit or 
in both. The heat will appear wherever there is resistance. 
If the poles of a battery or dynamo be short circuited, most 
of the heat will be developed in the battery or dynamo. 
If the external circuit be a thin wire of high resistance, it 
will grow hot while the generator will remain compara- 
tively cooL 



354 



ELECTRICITY AND HEAT, 



471. Joule's Law. — The quantity of heat developed 
in a conductor by the passage of an electric current is 
proportional : — 

(1.) To the resistance of the conductor. 

(2.) To the square of the strength of the current. 

(3.) To the time the current is flowing. 

A current of one ampere flowing through a resistance 
of one ohm, develops therein, per second, a quantity of heat 
which (or its mechanical equivalent) is called a joz/^^^^. It is 
equal to G.7373 of a foot-pound or to 0.24 of a lesser calorie 
(§ 579). A lesser calorie is, therefore, equal to 4.17 joules. 

These facts are concisely stated by the following equa- 
tion, known as Joule's Law: — 

H ^- C^Rt xO.24, 
in which ^represents the number of lesser calories ; C, the 
number of amperes ; R, the number of ohms and ^, the 
number of seconds. In other words, a current of one 
ampere flowing through a resistance of one ohm 
develops therein 0.24 of a lesser calorie per second. 

Foot-pounds =C^Rt x 0.737335. 

{a.) In investigating this subject. 
Joule used instruments on the prin- 
ciple indicated in Fig. 251, in which 
a thin wire joined to two stout con- 
ductors is enclosed within a glass 
vessel containing alcohol, into which 
a thermometer dips. The ' resist- 
ance of the wire being known, its 
relation to the other resistances may 
be calculated. 

Experiment l05.-^Send the cur- 
rent from a few cells through a 
chain made of alternate links of 
silver and platinum wires, The platinum links grow red-^hot while 




ELECTRICITY AND HEAT. 355 

tlie silver links remain comparatively cool. The explanation is that 
the specific resistance (Appendix K, [2]) of platinum is about six 
times that of silver and that its specific heat is about twice as great ; 
hence the rise of temperature in wires of equal thickness traversed 
by the same current is about twelve times as great for platinum as 
for silver. 

473. Heating Wires by the Current. — ^The 
resistance of metals increases with the temperature. Con- 
sequently, a thin wire heated by the current will resist 
more and more and grow hotter and hotter until it 
loses heat by conduction and radiation into the surround- 
ing air as rapidly as heat is supplied by the current. 
Thin wires heat much more rapidly than thich, 
TJze rise of temperature in different parts of a 
wire of uniform material but varying diameter 
(the current remaining the same) will he in- 
versely proportional to the fourth power of the 
diameters. 

{a.) Suppose a wire at any point to become reduced to half its 
diameter. The cross-section will have an area \ as great as in tlie 
thicker part. The resistance here will be 4 times as great, and the 
number of heat units developed will be 4 times as great as in an 
equal length of the thicker wire. But 4 times the amount of heat 
spent on \ the amount of metal will warm it to a degree 16 times as 
great (16 = 2*). 

(&.) A thin platinum wire, heated white-hot by a current, is some- 
times used in surgery, instead of a knife, as it sears the ends of the 
severed blood vessels and thus prevents hemorrhage. Platinum is 
chosen on account of its infusibility, but even platinum wires are 
fused by too strong a current. Carbon is the only conductor that 
resists all attempts at fusion (§ 466). 

(c.) Sometimes stout conducting wires are laid from a battery at a 
safe distance to a fuse connected with a blast of powder or other ex- 
plosive. In the fuse^ is a thin platinum wire, forming part of the 
electric circuit. The fuse is ignited by heating the platinum wire 
by sending the current through it. Such methods are frequently 
used in the operations of both peace and war. 



356 ELECTRICITY AND MOTIVE POWER. 

dTS, Electric Motors. — An electric motor is a 
device for converting the energy of an electric cur- 
rent into motive power by means of electro-magnets. 
Illustrative apparatus of this kind may be found in many 
school laboratories or will be gladly supplied by dealers in 
philosophical apparatus. But the best electric motors are the 
now common dynamo electric machines or slight modifica- 
tions thereof. Such ^^electro-magnetic engines'^ are rap- 
idly coming into use for operating sewing machines and 
other light machinery, the current being supplied indirectly 
by a storage battery or directly by a voltaic battery or 
dynamo. Some "Electric Light and Power Companies" 
now run such motors on their arc light circuits^ selling 
current to some for power and to others for light. In 
many cases where it is undesirable to use a steam engine, 
an electric motor may be made available. Such motors, 
up to the capacity of 40 H. P., are now in the market. 
Some of them have been successfully and economically 
used in propelling street railway cars. 

474. Electric Transmission of Power.— A 

water fall, perhaps at a point not easily accessible, may be 
made to turn a turbine or other water wheel, which shall 
drive a dynamo, which shall generate a current, which 
shall be carried by wire to some available point and there 
converted into mechanioal power again by means of an 
electric motor. Thus, an otherwise waste water-power 
may be made a source of profit. The scheme of thus dis- 
tributing part of the power of Niagara over the State of 
New York has been seriously considered. It may be pos- 
sible (as a profitable commercial undertaking) to burn 
cheap fuel at the coal mine for running large stationary 



ELECTRICITY AND POWER. 357 

engines and thus deliver the power to consumers at great 
distances. 

475. The Watt. — The electric unit of power (rate 
of doing work) is called a watt. A watt is the amouizt 
of power conveyed by a current of one ampere 
through a difference of potential of one volt. It 
equals (10~i x 10» —) lO'^ ergs or yjg horse-power, 

W=C X E = -^ = C^R, 

in which W equals the number of watts; (7, the number 
of amperes ; E^ the number of volts and Ry the number 
of ohms. 

For example, if the diflference of potential (Appendix M, 
[4 a.]) between the terminals of an arc lamp that is sup- 
plied with a ten ampere current be 45.8 Yolts, how much 
of the power used in driving the dynamo is consumed in 
the lamp ? 

W^C X E=l{) X 45.8 = 458, the number of watts. 
458 -7- 746 =: 0.614, the number of horse-powers. 

(a.) The formula TF— (7 x j^ is determined by the definition of 

E 
the watt. From Ohm's law, we see that (7 = — . Substituting this 

R 

E E'^ 

value of (7, the formula becomes W= — x E= — , as above. This 

R R 

shows that the power varies as the square of the E. M. F, when the 

resistance remains constant y or that the power 'caries inversely as the 

resistance when the E, M. F. remains constant. 

(6.) W =iC X E. But E = G B. Substituting this value of E, 

the formula becomes W— C x G E= G^BySiS above. This shows 

that the power varies as the square of the current when the resistance 

remains constant or that the power varies as the resistance when the 

current remains constant. 



358 ECONOMY OF CONDUCTION. 

476. Relation of Conductors to E. M. F.— 

This subject may be well studied by means of an example. 
The energy of a ten ampere current with an E. M. F. of 
fifty volts is equal to that of a five ampere current with 
an E. M. F. of one hundred volts. 

W= C X ^=10 X 50 = 100 X 5 = 500. 

These equivalent currents (500 watts each), flowing through 
similar wires, will develop widely different quantities of 
heat. If we take any convenient wire, say one of fifteen 
ohms, the heat developed in each case will be as follows: 

H= C^ X Ht X 0.24 (§ 471.) 

10^ X 15 X 0.24i=360, the number of heat units jt?er second. 
53x15x0.34= 90, " 

In other words, the same electric energy develops only 
one-fourth as much heat with the current of high electro- 
motive force as it does with the current of low E. M. F., 
the same wire being used. It is easily evident that a great 
saving in the cost of conductors may be made possible by 
the use of currents of high E. M. F. (See § 474.) But 
such currents are more dangerous to handle and require 
careful insulation and special precautions to lessen the 
risk of serious accident. 



ELECTRICITY^ HEAT AND WORK. 359 



Exercises. 

1. What shorter name may be given for a volt-ampere ? 

2. What electrical horse-power is required to send a current of 10 
amperes through 10 arc lamps (in series) each having a resistance of 
4.476 ohms ? Ans. 6 H. P. 

3. How many joules will be developed per minute by a 10 ampere 
current in a lamp of 4.42 ohms resistance ? Ans. 26520 joules. 

4. How many calories will be developed in a 40 ohm incandescence 
lamp by the passage of a current of 1.2 amperes through it for a 
minute? Ans. 82.944 calories. 

5. Find the mechanical equivalent (in foot-pounds) of the work 
done by a 5 ampere current working for a minute against 100 ohms 
resistance ? Ans. 110600J foot-pounds. 

6. A 30,000 watt dynamo develops an E. M. F. of 3000 volts. 
What is the current strength ? Ans. 10 amperes. 

7. One coulomb of electricity, in passing from A to B, does one 
joule of work. What is the difference of potential between A and B ? 

8. The difference of potential between the two terminals of an 
arc lamp was found to be 37.7 volts. A 25 ampere current was 
passing through the lamp. What is the power consumed in the 
lamp? Ans. 942.5 watts, or 1^ H. P. 

9. A certain Edison incandescence lamp has a resistance of 125 
ohms. The difference of potential between the terminals of the 
carbon is 110 volts, (a.) What is the current strength? (6.) What 
amount of heat is developed in the lamp per second ? 

Ans. (a.) 0.88 ampere; (&.) 23.23 lesser calories. 

10. A Grove cell has an E. M. F. of 1.9 volts and a resistance of 
0.4 ohm. Its plates are joined, first, by a 3 ohms wire ; second, by 
a 30 ohms wire, (a.) What is the current in each case ? (h.) What 
amount of heat is developed in the cell in each case ? 

i{a.) .559 amperes in first case. 
.0625 " " second case. 
(p.) .125 joules " first case. 
.00625 '' '' second case. 

About 80 times as much. 



360 



ELECTRICITY, HEAT A^D WORK. 



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REVIEW. 361 



Review Questions and Exercises. 

I. {a.) Give the laws for pressure of liquids and ih.) explain each 
by some fact or experiment. 

3. {a.) What is a natural magnet? (6.) An artificial magnet? 
(c.) How does a magnet behave toward soft iron? (d.) How does 
one magnet behave toward another magnet? 

3. Give the facts in regard to the variation of the magnetic needle. 

4. {a.) What are conductors in electricity ? ih.) In what ways may 
electrical separation be effected? 

5. {a.) What conditions in the construction and erection of light- 
ning-rods are necessary to insure safety from lightning ? (&.) Give 
the elements of a simple voltaic cell and (c.) the electric condition of 
those elements within and without the liquid. 

6. {a,) A body weighs at the surface of the earth 1014 lb. ; what 
would it weigh 1200 miles above the surface? (&.) Give the velocity 
of water issuing from an orifice, under a head of 81 feet, (c.) If 5 
quarts of water weigh as much as 7 of alcohol, what is the specific 
gravity of the alcohol ? 

7. Find the kinetic energy of a 25 lb. ball that has fallen 3600 feet 
in vacuo. Arts. 90,000 foot-pounds. 

8. Give the fundamental principle of Mechanics and illustrate its 
application by one of the mechanical powers. 

9. {a) Over how high a ridge can you continuously carry water 
in a siphon, where the minimum range of the barometer is 27 inches ? 
(5.) Explain. 

10. (a.) What is specific gravity? (5.) How do you find that of 
solids? (c.) What principle is involved in your method ? 

II. (a.) How much water per hour will be delivered from an 
orifice of 2 inches area, 25 feet below the surface of a tank kept full 
of water, not allowing for resistance? (&.) Give the law of magnetic 
attraction and repulsion. Ans, 14,998.44 gal. 

12. {a.) State what you have been taught concerning the dipping 
needle, (b.) Define and illustrate magnetic induction. 

13. {a.) Give the law of electric attraction and repulsion and illus- 
trate by the pith-ball electroscope. (&.) Define conductors and non- 
conductors, electrics and non-electrics, (c.) Illustrate by an example 
of each. 

14. {a.) Explain (by figures) electric induction, (b.) Explain the 
charging of a Leyden jar. (c.) When charged, what is the electric 
condition of the outside and inside of the jar ? 



362 rmvTeW. 

15. (a.) Give the sources of atmospheric electricity and (&.) the 
effects of lightning. 

16. {a) What is the effect of breaking a magnet? (b.) Grive a 
theory of magnetism that is competent to account for the properties 
of magnets, broken or unbroken. 

17. {a.) How do soft iron and tempered steel differ as to suscep- 
tibility to magnetism ? (b.) Describe one method of magnetizing a 
steel bar. 

18. The influence of the earth's magnetism upon a magnetic 
needle is merely directive, {a.) Explain what this means. (6.) 
Show why it is so. 

19. {a) What is meant by electromotive force? ih.) Describe 
Grove's battery and its mode of action, (c.) Why are battery zincs 
generally amalgamated ? 

.20. {a.) Describe Oersted's apparatus and (&.) tell what its use 
teaches, (c.) Describe the construction of the astatic galvanom- 
eter. 

21. {a.) Describe an electro-magnet and (&.) tell what its advan- 
tages are. (c.) State the principle of the electric telegraph. 

22. {a.) Describe a Ruhmkorff's coil and (&.) explain its action. 

23. ia.) Define electrolysis and electrolyte, (b.) Describe the elec- 
trolysis of water, (c.) Give a clear account of some branch of elec- 
tro-metallurgy, {d.) What is meant by the terms electro-positive and 
electro-negative f 

24. {a) Define physics, (b.) Name and define the three conditions 
of matter, (c.) What do you understand by energy ? {d.) Explain 
what is meant by foot-pound. 

25. {a.) What condition of the atmosphere is desirable for experi- 
ments in frictional electricity? (5.) Why? (c.) How gould you 
show, experimentally, that there are two opposite kinds of elec- 
tricity ? 

26. (a.) Describe the experiment with Faraday's bag and (&.) state 
what it teaches, (c.) Describe the dielectric machine and {d.) ex- 
plain its action. 

27. In an air-pump, the capacity of the cylinder is one-fourth that 
of the receiver. Under ordinary atmospheric conditions, they to- 
gether contain 62 grains of air. Find the capacity {a.) of the re- 
ceiver, (&.) of the cylinder. After 5 strokes of the piston, (c.) how 
many grains of air would be left in the receiver? What would be 
its tension {d.) in pounds per square inch ? {e.) In Kg. per sq. cm. f 
(/.) In inches of mercury ? Ans. {e) 327.68^. 

28. {a) Supposing we had two Leyden jars, one charged on the 
inside with positive electricity and the other with negative on the 



REVIEW. 363 

inside ; the two jars being insulated, can the jars be fully discharged 
by connecting the inner coats? (b.) Give reasons for your answer. 

29. In a vessel having the dimensions of a cubic foot, sulphuric 
acid (sp. gr. = 1.83) stands eight inches high ; give the pressure on 
the bottom and each side. 

30. The lever of a hydrostatic press is six feet long, the fulcrum 
being at the end and one foot from the piston rod. The diameter of 
the tube is one inch ; that of the cylinder, ten inches. The x)ower is 
25 lb. ; give the effect. (See Appendix A.) 

31. Find the joint resistance of three conductors of 10, 12 and 18 
ohms arranged in multiple arc. Ans. 4.18 ohms. 

32. {a.) Define equilibrium and its kinds, (b.) Give examples, 
(c.) How does the centre of gravity of any system, acted upon by 
an exterior force, move? {d) Give an example. 

33. {a.) Figure a simple barometer. (6.) Explain why the mer- 
cury stands above its level, (c.) What atmospheric pressure will 
sustain a column of mercury 24 inches high ? 

84. {a.) How is it proved that air has weight? (&.) What is the 
weight of air in a room 30 feet long, 20 feet wide and 10 feet high. 

35. When a 1000 gram flask, containing 700 g. of water, was filled 
with the fragments of a mineral, it weighed 1450 g. Give the 
specific gravity of the mineral. Ans. 2.5. 

36. A tank measuring 1 meter each way is filled w^ith water : 
what will be the pressure on the bottom and sides ? 

37. {a.) What is meant by kinetic energy? (6.) By potential 
energy ? 

38. Two inelastic bodies are moving in opposite directions, one 
weighing 31 grams and having a velocity of 24 meters per second, 
the other weighing 22 grams and having a velocity of 18 meters 
per second: what is the united energy {a.) before and (6.^ after 
impact? Ans. {a.) 1.27; (&.) 0.137 kilogrammeters. 

39. Regarding the same bodies as moving in the same direction, 
what would be the energy (a.) before and (6.) after impact ? 

40. {a.) Draw a simple figure showing the essential parts of an 
air-pump and {b.) explain the process of forming a vacuum, (c.) If 
Ihe capacity of the barrel be ^ that of the receiver, how much air 
will remain in the receiver at the end of the fourth stroke of the 
piston ? and (d.) what would be its elastic force compared with that 
of the external air ? Ans. {d.) |f|. 

41. The current of a Grove's battery with a certain resistance in the 
circuit is known to be i ampere. Passing this current through a 
sine galvanometer, the coils had to be turned 9° to bring them par- 
allel with the needle. (See Appendix L [2]). Some of the resistance 



364 REVIEW. 

being removed, it is found that the coils have to be turned 70° to 
bring them parallel. What is the current in the latter case ? 

Arts. 1 ampere. 

42. If a positively electrified ball be hung at the centre of a room, 
its charge will attract an equal amount of — electricity to the walls 
of the room. To what common piece of physical apparatus is this 
arrangement analogous ? 

43. What length of No. 6 pure copper wire (B. & S.) will have a 
resistance of 1 ohm? (See Appendix I.) Ans. 2433.09 ft. 

44 If a foot of pure copper wire weighing 1 grain has a resistance 
of 0.2106 ohm and 20 feet of commercial copper wire weighing 150 
grains has a resistance of 0.613 ohms, what is the percentage con- 
ductivity of the latter as compared with pure copper ? 

Ans. 91.6 per cent. 

45. Sketch an arrangement by which a single line of wire can be 
used by an operator at either end to signal to the other ; the condi- 
tion of working being that whenever either operator is not sending 
a message, his instrument shall be in circuit with the line wire 
and out ^/circuit with the battery at his end. 

46. Calculate, by Joule's law, the n amber of heat units developed 
in a wire whose resistance is 4 ohms when a steady current of 0.14 
ampere is passed through it for ten minutes. Ans. 11.2 units of heat. 

47. A dynamo has an E. M. F. of 839 volts and an internal (or in- 
terpolar) resistance of 10.9 ohms. Find the current strength when 
the external resistance is 73 ohms. Ans. 10 amperes. 

48. I have 48 cells, each of 1.2 volts E. M. F. and each of 2 ohms 
internal resistance. What is the best way of grouping them together 
when it is desired to send the strongest possible current through a 
circuit whose resistance is 12 ohms ? Ans. Group them three abreast. 

49. The current from a certain dynamo (E. M. F. =r 839.02 volts) 
was sent through a series of 16 arc lamps each having a resistance 
of 4.51 ohms. The line wire had a resistance of 0,8 ohm. The 
current measured 10.04 amperes. What was the resistance of the 
dynamo? Ans. 11.32 ohms. 

50. Immediately after the discharge of a Leyden jar, the potential 
of its knob is zero. It, however, begins to rise and soon has a value 
that is a considerable part of the potential before discharge and with 
the same sign. Explain this. 

51. What three varieties of energy appear when a Leyden jar is 
discharged ? 

52. How many heat units (calories) will be developed by a 10 am- 
pere current flowing through a coil of 50 ohms resistance in a quar- 
ter of an hour ? Ans. 1,080 calories. 




REVIEW. 365 

53. A current of 9 amperes worked an electric arc light and on 
measuring the difference of potential between the two carbons by 
an electrometer it was found to be 140 volts. What was the 
amount of power absorbed in the arc? Ans. 1.69 H. P. 

54. If the cells represented in Fig. 252 
have each an internal resistance of 4 ohms, 
what is the resistance of the external cir- 
cuit, G, if the battery is working at its gi-eat- 
est possible efficiency ? Ans. % ohms. 

55. The same strength of current that 
will heat an inch of platinum wire to white- 
ness will similarly heat a yard of the same 
Avire. Explain why it is necessary to use 
more cells thus to heat a yard than it does 
to heat an inch of the wire. 

56. Five Daniel cells, each with an E. M. PiQ 252. 
F. of 1.1 volts and an internal resistance of 

2.2 ohms are joined in series. The external circuit consists of 16743 
feet of No. 14 copper wire (B. & S.) (See Appendix I.) {a) What 
is the resistance of the external circuit? (6.) What is the current 
strength? Ans, (6.) 0.1 ampere. 

57. Show that with an unlimited number of cells like that just 
described, joined in series, the current cannot exceed 0.5 amperes. 

58. What is the total energy of the current of the dynamo, operated 
as described in Exercise 1 , page 349 ? Ans \ ^^^'^- ^^ watts. 

(11.28 horse-power. 

59. Explain the use and construction of a relay. 

60. Suppose 1000 incandescence lamps to be placed parallel in the 
circuit of a dynamo. Each lamp has a hot resistance of 50 ohms 
and requires a current of 1 ampere, {a.) What will be the current 
strength developed by the dynamo ? (&.) What is the resistance of 
the lamp circuit, ignoring the resistance of the leading wires ? (c.) 
What is the necessary difference of potential between the binding 
posts of the dynamo ? {d) If the resistance of the dynamo itself is 
0.005 ohm, what is the total E. M. F. ? {e.) How many watts will be 
expended in each lamp? (/.) If 500 of the lamps be turned off 
(open circuited), what will the resistance of the lamp circuit become ? 
{g) If the E. M. F. of the dynamo be kept constant by change of 
speed or otherwise, what will be the current developed by the 
dynamo with the 500 lamps? (Ji.) What will be the current then 
supplied to each lamp? 

Ans. (a.) 1000 amperes; (b.) 0.05 ohm ; (c.) 50 volts ; (d.) 55 volts ; 
(e.) 50; (/.) 0.1 ohm; (g.) 523.81 amperes; (h.) 1.047 amperes. 



366 REVIEW. 

H02!^^0RARY PROBLEMS. 

1. Two incandescence lamps with resistances of 16.9 and 32 ohms 
respectively were joined in series with a series of 40 similar voltaic 
cells having a total resistance of 20 ohms. The current measured 
1.16 amperes. What will be the strength of current that a series of 
60 such cells will send through a series of four lamps having resist- 
ances of 16.9, 32, 20 and 16 ohms respectively ? 

Ans. 1.043 amperes. 

2. What would have been the strength of current in this case if 
the area of the battery plates had been doubled, all things else re- 
maining the same ? Ans. 1.2 amperes. 

3. It required 15.3 H. P. to drive a certain dynamo that had a 
resistance of 10.5 ohms and developed a current of 10 amperes 
through an external resistance of 73 ohms. (The " duty " of a dy- 
namo is the ratio of the total electrical energy developed to the 
work performed in turning tlie armature in the magnetic field). 
What is the duty of the dynamo in question ? Ans. 73 per cent. 

4. The " commercial efficiency " of a dynamo is the ratio of the 
electrical energy appearing in the external circuit to the work per- 
formed in turning the armature in the magDetic field. What is 
the commercial efficiency of the dynamo above mentioned ? 

Ans. 64 per cent. 



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